Very interested what Dylan was even doing at the time. Don't think I've seen a witness describe it.
If I had to guess, I would say Dylan would kill himself. Wanted to anyway, massacre seems in part because he's afraid to die alone, and the only person he might have cared about letting down is dead. He seemed to be more about the killing students than killing cops, so he may have entered and fired a few rounds, but he probably thought he at least killed Lance.
I think Eric was more into killing cops (I think that's why he's at the top of the stairs where he can't really shoot people fleeing the cafeteria but Dylan goes down them and could have easily, to cover Dylan's back and fight the cops from the elevated position. Not TEC-9 short range "wrath" against the students but carbine long range "natural selection" living dangerously in a firefight with police.) I also think Dylan was more the mastermind (why he got to shoot students and had the TEC-9 which Eric seemed to dream about, for one) and Eric the follower contra the usual story. So, if I were to guess, I think Eric would have sought revenge for his friend against police, and doubled-down on going after them until he was killed.
So in short, shortly after there would be suicide for dylan and suicide by cop for Eric given that scenario, as best I can guess.