I'm not sure if it's my crummy Tascam US-122 MKII or Sonar. If I load two RXP plug-ins it will cause audio distortion (actually sounds more like overloading the output). With both plug-ins loaded, I can mute one and the distortion goes away, but with both unmuted it distorts. Distortion occurs with both headphones and speakers.
 
Does anyone else find this to be true? I'm patch to (X3e build 352). 
 
Regarding my crummy Tascam, if I upgrade to the latest patch my device no longer works, so I'm stuck on patch 2.2.4.0 (driver date: 6/19/2010 <-- 2010!! )

What's your cpu usage like when you have two loaded? You may have to change your buffers setting...
 
 

CPU is about 3%. Under Midi > Prepared Using = 250 Milliseconds Buffers.
I made changes to handful of settings in Preferences, but nothing seemed to help; I set them back to what they were. Stuff like changing Drive Mode (I have it as ASIO/Triangular), tried WDM, WASAPI, and even MME. 
 
I removed my Tascam and used the on-board sound, and still received the distortion, so it seems it's related to Sonar.

known issue with rxp:
http://forum.cakewalk.com/Sonar-RXP-Crackles-with-multiple-instances-m2118982.aspx
http://forum.cakewalk.com/Noise-when-more-than-one-instance-of-RXP-m1946072.aspx
the second link suggests using jbridge to fix it
hth

Aah, I use jBridge by default instead of bitbridge, maybe that's why I don't get those issues...
 

Thanks Paul! [Edit]
 
I had tried to use jbridge due to an issue with X2 and Dimension Pro, which X3 resolved the issue; I never did get jbridge to work correctly. I see the check jbridge check box in my RXP plug-in properties, but it's disabled (disabled for all plug-ins). I'm not sure I need to install jbridge, but installed the demo version, which enabled the check box, and I enabled jbridge for RXP. This solved the issue, but caused Sonar to hang after I loaded the plug-in, and also raised an error. I uninstalled jbridge, removed the RXP directory that jbridged created and tried two RXP's again and they worked.
 
I reviewed the RXP plug-in properties and jbridge was still checked, but still disabled (unable to uncheck it). With jbridge uninstalled, the hang issue didn't occur as well as no error, but it's working now. 
 
Thanks also for finding the threads, which I wasn't able to locate.
 
[Edit - I rebooted and the problem is still occurring. So I need install jbridge again and try to make it work with that. The jbridge readme.txt says, "*Currently, the following VST hosts provide built-in jBridge support - Cakewalk Sonar 8.5.x or above". So I would assume I don't need to install it - right? If that's the case any reason why jbridge is disabled? I would assume I might need to reinstall X3?]