It also depends on the fish. One of my bettas can NOT handle being able to see another betta. He had plenty of places to hide in my divided 5 gallon, but still chewed his fins down to nubs from the stress. He's in his own 2 gallon now, because after the first two days he still hadn't settled down and I couldn't stand to see him so stressed.
YMMV, but like koimaiden said, give them lots of hiding places and watch them closely.
Of course, Norman (the tail biter) is really high-strung, so it's probably just him. ;)