I just tried it last night before I replied and it worked fine for me, here's what I did:
-close all apps
-select the audiobus input app
-select cubasis as the output app
-in audiobus, click on the faded input app to launch it
-go back to audiobus and click on the faded cubasis icon to launch it as well
-in cubasis, you should now have the input app setup as a new track
-go back to audiobus and click on the + button on top of the input app
-go back to cubasis now and another track should have been created and set up with the new input
This should work, but if it doesn't try different apps as the input, since it could be a problem with the app possibly.
In my tests I used Animoog and Arturia SEM.
Other than that, does your input app support Inter-app Audio? That could be a workaround...
iPad Air + iOS 7.1 + Presonus Audiobox 1818VSL