Kage49, are you sure that Cubase and MIR don't try to access the same audio port? You have to tell both applications that they _must not_ touch the audio ports used by the other program. In Cubase, it's done in the VST-connections (F4 by default); in MIR it's under Audio Settings / Audio Routing.
Apart from that: What are the system-wide latency settings of the RME? And how many buffers does MIR use additionally ("Buffer Multiplier" ... it's under the Preferences / Advanced)?
Kind regards,
/Dietz - Vienna Symphonic Library