Whenever I have 2 back-to-back matches (15 mins apart), I know that time will be short between games. What I then do is, place my 11 for the first match and then make sure my reserves are fully rested and have high morale. Then, if at the end of first game, I have tired players and not enough time to change them, I just let the same team start the second match, but make substitutions early on if needed.

Of course, I'm risking being short-handed if a player gets injured late on in the game and I've used all my subs, but so far this has worked pretty well for me.