I dont understand exactly what you want to do...
Do you want to run audio from the 428 into the motu, use them both to bring audio in at the same time, or just use the 428 as a control surface for cubase?
The first one, yes you could do that but I dont know why you would need to
The second - eh, could cause a lot of problems, I've had bad results trying two interfaces at once before
The third, yeah, you can do that, I'm going to continue to use my 428 as a control surface now that I've recently added a motu 828 to my setup.