What is the offspring of two Demi gods called?

The two offspring of demigods depends on how powerful the demigod is if they are both demigods of really powerful gods it is a trigod if there is one powerful god a quadgod and if they are both from minor or non-powerful gods they arent really demigods anymore

I'm not sure from where that answer comes, but it isn't actual mythology. Divine heritage was a literary device, and a means for important men to add legitimacy to their power. Julius Caesar claimed divine heritage as a descendant of Aeneas, whose mother was Venus/Aphrodite. That said, Aeneas had a son, Aascanius, whose mother was Creusa, daughter of Priam and thus a descendant of Zeus. He wasn't called a demigod or anything other than "king" (of Alba Longa).

Technically demigod means "half god". They didn't deliniate beyond that because there was no need to. They weren't creating a new race - they were trying to make themselves more important.