I'm with Grumpy on this one, it's highly doubtful you'll run into any problems running two arduino's where one arduino runs fine.
I think the regulator issues are based more on the voltage... I've ran nearly 400 ma for.. I don't have an exact time.. but long enough to get warm. I'm guessing like 20 minutes ish? Was testing some servos.. and there was almost no heat dissipation.
But once I plugged the Arduino into a 9v Regulator (regulated, constantly puts out about 8.75 when it doesn't have a load) and did the same test.. got fairly warm very quick.
Just put the 5v line to the 5v line on the arduinos, and the ground to the ground. That'll power them both. Assuming one actually has power of course. :D
Just curious though, what's the reasoning behind trying to connect two boards? (they are boards right? not the chip alone?)