Can not run 2 Diablo 3's successfully

Hey guys. Recently I got myself a second account to try out multiboxing but I can not seem to run diablo 3 twice... I can launch both accounts. Log into each of its characters but the moment I try to join a game one of the screens will always grey out and refuse to launch. I am only using 600/4000 Ram.... I have quad boxed games like World of warcraft and Swotr but not diablo... It does not make sense. You would think a game like diablo takes barely any CPU to run... Could it be a result of my computer itself?

I am using high performance with 100% processor
my specs


AMD Athlon X4 635 2.90Ghz
4Gb Ram
Nvidia Geforce Gtx 460


if it means anything I can not even LiveStream diablo 3 properly without getting huge FPS issues... While on the other hand I could stream a Quad box Dual monitor stream at 1080P For wow...

[lax]

Your computer is probably fine.

There's a D3Debug.txt file you can show me that might shed some light on this problem, see http://isboxer.com/forum/viewtopic.php?p=10314#p10314 for instructions