Short answer: RTAS does not allow "virtual tracks" to share the same voice (disk buffer). Long Answer: RTAS plug-ins process any audio data residing in the disk buffer. If another track was using the same voice, DAE would have to "switch off" the RTAS processing at the sample boundary and switch to the other track. It makes the code to add RTAS plug-ins more difficult and thus, more likely to have bugs. As a result, we opted to keep things more simple and restrict you from sharing voices when using RTAS plug-ins.