I've heard of people doing it. I try to avoid it myself because cash game play tends to be somewhat different from tournament play. Strategies tend do differ from one to the other. If I multitable, I rather it to be games of the same type. For instance, I prefer to play three or four NL S&Gs or multiple cash games at a time then to mix NL games with limits games, or be playing PLO and NLH games at the same time, or even play an MTT while I'm involved in a cash game. Note that I did say prefer. I won't claim never to have done it, or not to do it from time to time still.