Search multi-indices(having some same field name), how to search cross indices

Elastic Stack Elasticsearch
1

Hi, Have some question on cross multi-indices search (using version 6.3)
Say, two indices (on purpose, no merge) with one common field name subject
Index1 field: subject, .........
Index2 field: subject,........

I'd like to query both indices on field subject, but boosting different weight during query time:
on one index:
POST _search
{
"query": {
"match" : {
"subject": {
"query": "demo",
"boost": 2
}
}
}
}
How do I refer to field under different index ?

Any idea/help is highly appreciated.

Sam

2

Looks like index boosting is sort of what you're looking for?