What does Elasticsearch do when multiple indices share the same alias?

I mean exactly what I asked, I was wondering how elasticsearch will manage multiple indices with the same alias, will this be a way of load balancing or will elasticsearch pick one index randomly?
Thanks to all of you.

It'll query both when you query and it'll refuse to write to the index if you try write to it.

Isn't that to expensive if ES have to query both indices at the same time? Why would I query two indices with the same structure instead of querying just one? Could you clarify my doubts please?