While DLR has gotten more formalized with MM by adding MM days onto their online calendar, they have not done this for TTMM. So if they ever were to move it around it would be hard to know.
With that I doubt grad nights would impact TTMM. TTMM does not open before regular park hours. It just opens before TT hours. So there is no special hardship to park workers in getting TTMM going after a grad night.
As I've seen in tha past. Yes it can happen on the same day. Also sometimes DL opens later due to special events, and has resulted in DL opening at the same time as the TTMM. It appears that TTMM is inflexible and will occur despite what else is happening that day.