To my knowledge when you talk about the plugin version of Ctrlr it does not matter if you have a multiclient capable midi interface. The DAW and the Ctrlr plugin won’t ever be able to use both the same port.
As I have no multi client midi interface I can’t test it by myself, but this contradicts this post:
Windows XP/ Ableton 8.1
Midiman midisport 2X2: supports multiclient (selecting the midi out in ableton preferences as in the plugin)
RME Fireface midi: does not support multiclient. (selecting the CTRLR midi out will cause an error openDevice hardware failed if it is also selcted in ableton midi preferences))
It’s not a Windows port sharing problem. I definitely CAN share ports inside Cubase. I am only using one ‘app’ – Cubase. It’s just that Ctrlr (as a plug-in – I’m not running it separately) cannot access any port that is referenced in Cubase’s MIDI Device Manager. If I remove the reference then Ctrlr accesses the port just fine. Removing the port in Cubase isn’t an option anyway – I then wouldn’t be able to play the instrument in Cubase.
If I’m not mistaken, you’re sharing ports INSIDE the same app (Cubase). Of course this is possible without multi client drivers. But Ctrlr, even as a plugin, has to manage his own ports, independent of the host, so consider it as another app, just as when you use Ctrlr as a stand alone app.
Can you use Cubase and Ctrlr standalone at the same time using the same ports?