Nuendo & Cubase running simultaneously (not just installed) on one computer at different sample rates:
It works perfectly.
I've been testing it for two days without a single hickup under full load, various session configurations, and irrational moves.
As a side note, I briefly tested UAD and Powercore cards that were available, and they also work inside both applications at the same time. In theory, any hardware powered plug-in should work fine.
Besides the two software licenses, a reminder again that in order to run each application at a different sample rate, you need two audio interfaces that each has its own driver and control panel. For example, if you tried to do this with two RME HDSP cards it wouldn't have worked. Same for the Lynx PCI's etc.
On a different note, I'll have to do some very similar tests with PTHD as well, but that's irrelevant here. If you want details, feel free to PM. Along similar lines, it might be of use to some if I mention that you can't fully resolve this configuration (referring to clock and transport sync) because of the different sample rates involved. MTC or analog LTC along with MMC is as far as you can go to sync the trasports because digitally transmitted timecode can't work in this config, and there's also nothing you can do about syncing the ADDA's. I had extensive discussions with most top designers of sync devices; this info is valid, unfortunately, and it's got nothing to do with the DAWs themselves. If both software are set to the same sample rate then none of these sync issues apply and you're all good. Again, as this is quite irrelevant to this forum, if anyone wants details, PM.