Because of their length I think it's relatively easy to play multiple MTTs at once. SNGs can be a bit more hectic because it's not unlikely to have multiple games in the faster final stages.
I tried to play several tournaments at a time. and I did not like it, could not concentrate on the game. bluffed against whom it is not worthwhile and played a passive role against those where the bluff could take the bank. for myself, I decided that I will play one tournament at a time. but at full concentration. so I often get to be in the prize zone.