How to compare two documents from two different indexes using script

[JBone]

Hi, I am searching for similar fields in two different indexes. My goal is to if those fields match, then I know there is a correlation between those two documents.

I do search first with accountNumber, and I group them with another identifier, so that is the reason I am doing what I am doing in aggregations. this is just a part of that bigger query.

For example, in Index A1

{
"_id": 1,
"refNumber": 123
"uID": 456
}

and in index B2

{
"_id": 1,
"refNumber": 6456456,
"uId": 123
}

So in the above two documents, the 'refNumber' in A1 index is equal to "uId" in the document with index B2. So here is what I am trying

GET/A1, B2/_search
{
"size": 0,
"aggs": {
"correlatedDocuments: {
"terms": {
"script": {
"source": "if(doc['A1.refNumber.keyword'].value == doc['B2.uId.keyword].value ) { return 1}"
}
}
}
}
}

obviously that does not work. How to solve this kind of problem? it does not have to be Painless script. I just want to connect the documents based on those two key values in two different indexes.

The data remodeling is not an option. May be if I find a solution that needs a change, then I can push for the change but is not an easy task.

Any help would be appreciated