if they stay at the back after a lob they are giving you the net for free. First thing is only one of you should run back to get the lob, the other should stay at the net. The one who goes back should simply play a normal ball aimed at the opposing service line, forcing them react quickly because if it bounces before the line they'll have to play an uncomfortable half-volley, if it bounces behind the line they'll have to play with the wall. Both options give you plenty of time to reach the net while the ball is travelling so you can be there in time to volley (even if it's a block volley). No need to play that ball with high speed or power, if it goes deep (aimed at service line) and slowish, its better than deep and fast. Slowish gives you more time to reach the net next to your teammate, and speedier ball makes for a high bounce after the wall, you want a speed that bounces neutrally from the wall, not strongly off of it.
Never play a lob to opponents at the back btw. The sole purpose of a lob is passing the other players so you can rush the net. If they're at the baseline then A. you cannot pass them and B. the net is already yours for the taking, no need to lob.