If I'm playing 5/10 or below, I feel I can play ABC Poker and 8-table. (Here's a pic of me
doing that while wearing my cool poker pajama pants!).
It's not for everyone, and if playing higher than 5/10 I stick to 4 tables, with the other monitor using Pokertrackers Gametime Window.
I take a break every half hour, and run through Pokertracker, looking at any stats on players then add it to their notes. This helps a lot in regard to determining how to play hands.
I have to focus on this 100%, and thankfully I have an excellent memory for this sort of thing, plus the ability to completely forget about the dealt cards at a table from the prior hand.
And I've tried mixed games at the same time for a lark, and it just doesn't work. It requires you to stop and think, which while doing this you really can't.