My first-choice goalie was at 90% fitness and the backup was at 100%, with a back-to-back set of games coming up. I decided to start the backup in the first game, and he got replaced after 55 minutes of play (with a respectable 90%+ save percentage though). My starter came in and filled in for the remaining 5 minutes.
After the match, both goalies are as fatigued as they would be if either of them had played the whole game:
