You cannot overlap, but you can schedule them right next to each other.
If you want to try and squeeze them a little tighter, we had some luck with modifying our next fastpass to an earlier time once we used our first one. As soon as you go through the tapstile for one fastpass, that window is freed up to reschedule your next one.
For example, we had Big Thunder Mountain at 1:00 pm and Splash Mountain at 2:00 pm. Got in the fastpass line for BTM at 12:55 and while waiting to get on the ride, I was able to move Splash up to 1:15 pm using the mobile app.