RXP Questions, posiible misuse causing hangup

So I started using the RXP out of curiosity and love it.  I can't use more than one without crashing the program.  Is this common?

I find it a pretty easy way to make fast rythm tracks by using the matrix view and midi triggering various slices ran through the arpegiator coupled with some other synths.  I would like to be able to use two.

And since I don't have a tag.

Sonar X1 producer 64 bit
Quad core ADK, 8 gigs ram
Windows 7 64
M-Audio fasttrack
Akai MPk
Nintendo Miracle

Oh, and I like Moogs lately... any fav's you guys have in Dpro, Rapture or any upgrades I can get to expand?  Looking at the Arturia Moog Module... would that be compatible with Sonar?  Played with some of them listed... like the mini moog and the pedals... but kind of want more.