The fastest way to merge large dictionaries is to introduce an intermediate object that behaves as though the dicts are merged without actually merging them (see @Raymond Hettinger's answer):

from collections import ChainMap

class MergedMap(ChainMap):
    def __getitem__(self, key):
        result = []
        found = False
        for mapping in self.maps:
            try:
                result.extend(mapping[key])
                found = True
            except KeyError:
                pass
        return result if found else self.__missing__(key)

merged = MergedMap(myDict1, myDict2)

Whether it is applicable depends on how you want to use the combined dict later.

It uses collections.ChainMap from Python 3.3+ for convenience to provide the full MutableMapping interface; you could implement only parts that you use on older Python versions.

You could write a function to do this for you:

>>> d = {'word': 1, 'word1': 2}
>>> def set_key(dictionary, key, value):
...     if key not in dictionary:
...         dictionary[key] = value
...     elif type(dictionary[key]) == list:
...         dictionary[key].append(value)
...     else:
...         dictionary[key] = [dictionary[key], value]
... 
>>> set_key(d, 'word', 2)
>>> set_key(d, 'word', 3)
>>> d
{'word1': 2, 'word': [1, 2, 3]}

Alternatively, as @Dan pointed out, you can use a list to save the data initially. A Pythonic way if doing this is you can define a custom defaultdict which would add the data to a list directly:

>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> d[1].append(2)
>>> d[2].append(2)
>>> d[2].append(3)
>>> d
defaultdict(<type 'list'>, {1: [2], 2: [2, 3]})

Copyd = {'A': 1, 'B': 5, 'C': 2}
d.update({'A': 2})

print(d)

Copy{'A': 2, 'B': 5, 'C': 2}