Minutes per game:
Only Jaxson - 10:09
Only Spencer - 11:14
Both - 18:01
Neither - 0:36
So you are right, there were only 36 seconds per game where neither were on the court. But they did overlap significantly, over 18 minutes per game.
And also I was wrong, Jaxson replaced Spencer much more often than he replaced Trevin when he subbed in.