I always found the two to give me bizarre closed eye hallucinations. Like, pseudo-psychedelic mandalas made out of hands and faces and dicks, balls and cunts and stuff. Super wierd, half-realistic squishy hallucinations.
As for "lessening the roll" or whatever, all it ever did was fuck me right up even more. I mean, you're high on drugs, you do more drugs...
Ketamine doesn't block MDMA in any way chemically. Everyone is different, right, so nobody can really tell you when will be the right time to take what. You'll have to find that out for yourself.