Combining queries on different indices

This could be a very basic question. I am new to ES, couldn't get any good answer from my research. Hence posting it here.

I have 2 indices that need to be searched with 2 separate sets of queries(keywords)

eg:
Index 1 - "support-manual"; query based on terms "not able to reduce brightness"
Index 2 - "user-guide"; query based on terms "adjusting brightness"

Now, my question is - Is there a way to combine/merge these 2 queries into a single query which will give me a scored result considering both. Basically results from both queries should be ranked in a "combined" way.

Can you point me to the right documentation if this already a supported feature?

You cannot do scoring across two indices. So while you can query the two, each result set will be relative to the index.