Use the combine filter.

- set_fact:
    mergedobject: "{{ object1.params | combine (object2.params) }}"

You can use combine filter in 2.9. It's been in Ansible since 2.3. What you can't use in 2.9 is the option list_merge. In this case, you can iterate the list and merge the lists on your own, e.g. the play

- hosts: localhost
  vars:
    to_add: efg
    Variables:
      - Strings: [abc, xyz]
        Inputs: true
  tasks:
    - set_fact:
        V2: "{{ V2|d([]) + [item|combine({'Strings': _Strings})] }}"
      loop: "{{ Variables }}"
      vars:
        _Strings: "{{ item.Strings + [to_add] }}"
    - set_fact:
        Variables: "{{ V2 }}"
    - debug:
        var: Variables

does the job

  Variables:
  - Inputs: true
    Strings:
    - abc
    - xyz
    - efg

This template

shell> cat events.json.j2
{{ events|to_nice_json }}

and the tasks below

    - set_fact:
        events: "{{ {'LambdaFunctionConfigurations':
                     ([event_json_create.LambdaFunctionConfigurations.0] +
                      [event_json_existing.LambdaFunctionConfigurations.0])} }}"
    - template:
        src: events.json.j2
        dest: events.json

give

shell> cat events.json 
{
    "LambdaFunctionConfigurations": [
        {
            "Events": [
                "s3:ObjectCreated:*"
            ],
            "Filter": {
                "Key": {
                    "FilterRules": [
                        {
                            "Name": "prefix",
                            "Value": [
                                "keying_service/response/"
                            ]
                        }
                    ]
                }
            },
            "LambdaFunctionArn": "arn:aws:lambda:us-east-1:*******:function:xyz"
        },
        {
            "Events": [
                "s3:ObjectCreated:*"
            ],
            "Filter": {
                "Key": {
                    "FilterRules": [
                        {
                            "Name": "Prefix",
                            "Value": "staging/inbound/Source_Contact/ac/input_fia/"
                        }
                    ]
                }
            },
            "Id": "Eventtry",
            "LambdaFunctionArn": "arn:aws:lambda:us-east-1:******:function:abc"
        }
    ]
}

For example

    - debug:
        msg: "{{ locations|
                 json_query('[?state == `WA`].[name,state]')|
                 map('join', '-')|list }}"

gives

  msg:
  - Seattle-WA
  - Bellevue-WA
  - Olympia-WA

The same result gives the task below using Jinja2 filters only

    - debug:
        msg: "{{ _names|zip(_states)|map('join', '-')|list }}"
      vars:
        _locations: "{{ locations|selectattr('state', 'eq', 'WA')|list }}"
        _names: "{{ _locations|map(attribute='name')|list }}"
        _states: "{{ _locations|map(attribute='state')|list }}"

json_query issue (fixed in 2.10 and later)

There is JMESPath join. Unfortunately

    - debug:
        msg: "{{ locations|
                 json_query('[].join(`-`, [name,state])') }}"

fails

msg: |- JMESPathError in json_query filter plugin: In function join(), invalid type for value: Seattle, expected one of: ['array-string'], received: "AnsibleUnicode"

to_json|from_json workaround

Quoting from json_query: Add examples for starts_with and contains #72821

data structure returned from register variables needs to be parsed using to_json | from_json in order to get a correct result. Fixes: ansible-collections/community.general#320

    - debug:
        msg: "{{ locations|to_json|from_json|
                 json_query('[].join(`-`, [name,state])') }}"

gives

  msg:
  - Seattle-WA
  - New York-NY
  - Bellevue-WA
  - Olympia-WA

You need to investigate the extra parameter that the combine filter is offering you:

The filter also accepts an optional recursive=True parameter to not only override keys in the first hash, but also recurse into nested hashes and merge their keys too

{{ {'a':{'foo':1, 'bar':2}, 'b':2} | combine({'a':{'bar':3, 'baz':4}}, recursive=True) }}

This would result in:

{'a':{'foo':1, 'bar':3, 'baz':4}, 'b':2}

^{See: https://docs.ansible.com/ansible/2.9/user_guide/playbooks_filters.html#combining-hashes-dictionaries}

So in your case, you indeed have to combine you two dictionaries first, then, convert them in JSON:

- name: Combine existing S3 notifications with new notifications
      command: >
        aws s3api put-bucket-notification-configuration
          --bucket "{{ s3_notification.bucket }}"
          --notification-configuration '{{ s3_notification.configuration | combine(existing_s3_notifications, recursive=True) | to_json }}'
          --region "{{ aws_region }}"
      environment:
        AWS_ACCESS_KEY_ID: "{{ assumed_role.sts_creds.access_key }}"
        AWS_SECRET_ACCESS_KEY: "{{ assumed_role.sts_creds.secret_key }}"
        AWS_SECURITY_TOKEN: "{{ assumed_role.sts_creds.session_token }}"

Also mind that, if you are on version 2.10 already, you even have another extra parameter:

list_merge: Is a string, its possible values are replace (default), keep, append, prepend, append_rp or prepend_rp. It modifies the behaviour of combine when the hashes to merge contain arrays/lists.

^{See: https://docs.ansible.com/ansible/latest/user_guide/playbooks_filters.html#combining-hashes-dictionaries, containing nice examples for each values}

Do not use nested curly braces "{{ ... {{ item }} ... }}". Correct syntax is

all_users: "{{ all_users+ [{ 'UserID': item,
                             'UserType': 'TBD',
                             'UserName': 'TBD' }] }}"

Next, it's more convenient to submit a list of userids. Then the split filter is not necessary. For example the playbook

shell> cat playbook.yml
- hosts: localhost
  tasks:
    - name: Initiate User List
      set_fact:
        all_users: []
    - name: Add new JSON Objects to List
      set_fact:
        all_users: "{{ all_users +
                       [{ 'UserID': item,
                          'UserType': 'TBD',
                          'UserName': 'TBD' }] }}"
      loop: "{{ userids }}"
    - name: Debug
      debug:
        var: all_users

gives

shell> ansible-playbook -e "userids=['123456','654321']" playbook.yml
...
    "all_users": [
        {
            "UserID": "123456",
            "UserName": "TBD",
            "UserType": "TBD"
        },
        {
            "UserID": "654321",
            "UserName": "TBD",
            "UserType": "TBD"
        }
    ]

shell> ansible-playbook -e "userids=[123456,654321]" playbook.yml

gives (there are no quotes around the values of UserID)

    "all_users": [
        {
            "UserID": 123456,
            "UserName": "TBD",
            "UserType": "TBD"
        },
        {
            "UserID": 654321,
            "UserName": "TBD",
            "UserType": "TBD"
        }
    ]

Then use a dictionary to create the list of users. For example,

shell> cat playbook.yml
- hosts: localhost
  vars:
    my_users:
      123456:
        name: Alice Admin
        type: admin
      654321:
        name: Bob Business
        type: sales
  tasks:
    - name: Initiate User List
      set_fact:
        all_users: []
    - name: Add new JSON Objects to List
      set_fact:
        all_users: "{{ all_users +
                       [{ 'UserID': item,
                          'UserType': my_users[item].type,
                          'UserName': my_users[item].name }] }}"
      loop: "{{ userids }}"
    - name: Debug
      debug:
        var: all_users

gives

shell> ansible-playbook -e "userids=[123456,654321]" playbook.yml
...
    "all_users": [
        {
            "UserID": 123456,
            "UserName": "Alice Admin",
            "UserType": "admin"
        },
        {
            "UserID": 654321,
            "UserName": "Bob Business",
            "UserType": "sales"
        }
    ]