Use search results as base for another search


I have an index where i store comments for articles

Document is like this
first_post: keeps if that post was the first made by user

What i want to do is to find all the users that are matching field_X and field_Y and made their first post during a given period.
Base on that result i want to find the number of those users that have repost during another given period

First thing that comes in my mind is a two step search
So i have two questions:
a) I want to support large collection (thousands documents). How efficient is will be if i go with two step solution to add in second search thousands of term filters
b) Is there a way to achieve that in one query