You can't send multiple queries as query= qry1 qry2.. in a single HTTP API call. If you take a closer look to the response object:

$ curl 'http://localhost:9090/api/v1/query?query=up&time=2015-07-01T20:10:51.781Z'
{
   "status" : "success",
   "data" : {
      "resultType" : "vector",
      "result" : [
         {
            "metric" : {
               "__name__" : "up",
               "job" : "prometheus",
               "instance" : "localhost:9090"
            },
            "value": [ 1435781451.781, "1" ]
         },
         {
            "metric" : {
               "__name__" : "up",
               "job" : "node",
               "instance" : "localhost:9100"
            },
            "value" : [ 1435781451.781, "0" ]
         }
      ]
   }
}

You will see that "data" field contains only two keys (i.e. "resultType" and "result") and it isn't an array itself. So there is no chance that "data" will hold the result of multiple queries.

You can use the or operator, however this does not generalise as it ignores metric names. I'd suggest making multiple queries to the API.

#!/usr/bin/env python

import requests

prome_sql = "(node_memory_MemTotal_bytes - (node_memory_MemFree_bytes + node_memory_Buffers_bytes + node_memory_Cached_bytes)) / node_memory_MemTotal_bytes * 100"

response = requests.get('http://192.168.20.249:9090/api/v1/query',
    params={'query': prome_sql})
print(response.json()["data"]['result'])