[jre3132]

create a stereo track, you'll need the grabber tool selected (or if you have the smart tool enabled, just go to the bottom half of the track), highlight the audio on the 2 mono tracks, hold control then click and drag the audio to the stereo track.

holding control allows you to shift the audio up and down, but not side to side. so if you those 2 mono tracks line up with other tracks in the session, they won't get out of alignment.