So they can play together 80% of the game vs 60% (assuming 8 minutes of rest).
The same reason why hockey teams put their 4th line out. The goal is to give the rest of the team rest and they often times come in the game when the opponent puts out their 4th line. You are just hoping for no mistakes
I think the plan is for one of Harper or Bailey to be on the floor at all times. It just so happened vs SHU that Harper drew 2 fouls BEFORE Bailey had a chance to be rested for 2-3 minutes, so I suspect the 2 were out of the line-up at the same time because of circumstances, not design. That will happen at times.
The way it appears to be working out, the rotation of Harper and Bailey happens in stages in each half:
Stage 1: Starting line-up, both in together for about 4-6 minutes.
Stage 2: BAILEY gets the 1st rest, it looks like, for several minutes.
Stage 3: Bailey returns and Harper hits the bench for a rest Usually Bailey and another player come in at the same time - the other player being a guard to replace Harper ... i.e. Bailey does not sub for Harper directly).
Stage 4: Harper returns to the game, while Bailey stays in the game. This then remains the same for the rest of the half (both 1st and 2nd half).
There are times when Harper and/or Bailey, other than foul trouble, may get a quick rest, not at the same time, 1 minute before a scheduled TV time out, if there is a stoppage - or if Pikiell has plenty of time outs left he will often call a timeout at the 6 or 5 minute mark of the 2nd half after an RU make, and give one of these 2 stars a quick break.
In regards the overall rotation, it has varied over the course of the 1st 11 games. The last 2 games, with Davis starting, without going back and checking exactly, it FEELS like the rotation works something like this (I could be wrong, but think I am close):
1) Starters: Harper, Bailey, Williams, Ogbole, Davis.
2) Sommerville is generally the 1st man off the bench, replacing Ogbole - sometimes as early as the 17:30 mark of each half - a little later if Ogbole is visibly doing well - but NEVER later than the 16 minute mark, I don't think.
3) If Sommerville enters later, Derkack enters with him, for Williams, Davis or Harper, Williams and/or Davis rested first, I think ... but sometimes Derkack is a separate entry from Sommerville...
4) Hayes seems to be next in line, to rest Bailey. Martini may also come in around this time, in situational substitution for Sommerville.
5) Bailey returns, along with either Acuff or Williams, while Harper rests - and someone else of course must depart (Hayes or Derkack),
6) Harper returns - but Davis does NOT, unless there is a perceived need for defensive focus.