Strange problem dualboxing

Hello, I have an interesting Dualboxing problem that I was hoping someone could help me out with. I had recently been dualboxing in World of Warcraft with no problems, no lag on either side or anything, didnt even have to turn the graphics down. After an update I find I am no longer capable of doing it. I will open up one copy of wow and log in no problem. Then I will go out to open a second copy of it and I will just get a black screen, even though I can hear the login screen music playing. It basically locks up my computer and I have to restart. Any thoughts? 8|

I am currently running (I guess I should mention its on a iMac desktop)

2 GHz Intel core duo processor
2 GB of 667 MHz DDR Ram
Radeonx1600 (128mb of Vram) graphics card.

You running both in Windowed mode? If not, try that.

I was always running them both in windowed mode, I dont understand why it would work before, and not now.