I've never been a fan of having a pitcher throw in multiple games in one day either in a starting or relief role.
Most relief pitchers hate it when their managers warm them up to come into a game, then the starter gets out of the jam, goes out for another inning or two and then the reliever has to get warmeed up all over again.
From personal experience, I was brought into an MSBL 35+ World Series pool play game in a relief role two years ago and pitched 4 2/3 innings to get the win. We had a 90 minute gap between the end of that game and the start of the second game of our double header that day. I started the second game and went 4 1/3 before running out of gas. My arm was tired and lethargic the second game and my recovery time was easily triple what it normally was. I was 48 years old at the time.
I would strongly recommend not doing this with younger players whose arms are not fully mature. ( ie-growth plates fully closed).