How do I use a large input in a query?

I am try to write some query[ies] so that I find user who had done action A in one type of event and action B in another type of event. the time span is simply too long to use transaction command. Previously when I used to run a query for userid who did action A (the smaller set of the 2), manually make that list into a giant (userid1 OR userid2) and search for users who did action B. This would work when the output from action A was around 10k id. Now I have to do thing same thing, except the the number of user now increase to the hundreds of thousand range too impractical to do the manual approach. I tried using subsearch but that doesn't work well with large input data. can someone suggest a solution that scale well? thanks