They just scheduled the two of them on the same day based on how they entered camp and how ready they were to handle that amount of innings. Scherzer was ahead of deGrom in terms of pitch count, and deGrom wanted to have the routine of the starter. This is what happens when ... and this is key ...
There are fives teams playing each other in a small geographic bubble.
... That assures that once every five days, a team is off, completely, and that can lead to overlapping pitchers or a lot of back field assignments. The Cardinals had back fields going to get the innings for their starters. The Mets opted to put two on the same day and see what happens. Yes, it was a little theater. Yes, it was a lot of the schedule. Yes, it was a lot of where the pitchers were at that point in time. Scherzer wanted to go six, yes. That was his schedule.
deGrom has a stress reaction in his right shoulder. That is the same injury that Michael Wacha had to overcome and did so by changing his routine, taking control of his offseason rehab and his program, and then reclaiming a spot in the majors. He's with Boston now.