shuffle mode, in the upper left corner.
slip, grid, spot, and shuffle
you want shuffle, then use the grabber tool and just grab the tracks and the shuffle feature will butt them up together.
that's one way.....but if you have say 15 tracks of stuff and you just grab one track, it will butt up against the previous track and the other 14 are now out of sync with it, if that makes sense.
i assume you are just working with two tracks though (left right mixes)
just go back to "slip" mode once you are done and shuffle can create a lot of nightmares if you aren't careful.