I don't use cubase as well, but isn't there an option that you can select per track, which patch and which channel you are going to use?

In other words, you choose all midi outs in your cubase setup, and in your project (whatever it is called in cubase?) you select for each track your midi devices!
This is how I do it in sonar and how I did it it in all the versions of cakewalk.

(I use very often 6 to 7 external midi devices in one song without a problem)