Elasticsearch - matching between two index

Hey,

I have a big problem. I have to match two databases with tracks in
elasticsearch. So I uploaded them with logstash in two seperate index.
How can I match one index against the second?
So that I get for every record in the first index a result from the
second index.
Should I load the data in one single index?
Is there something I overlooked?

Thank you in advanced!

Chris