I hate to tell you this, but neither of those roses is going to be much of a climber in zone 5. They'll both have their canes killed each year by the cold and will have to start over. Real climbers in zone 5 are roses like the Canadian Explorer series of roses. I have John Cabot now, and I've ordered Quadra. Ramblin' Red and Brite Eyes are also supposed to be cane hardy in zone 5.

As for your planting, each rose needs its own hole. You can have both a rose and a clematis on each trellis. If your drainage isn't good you can build a raised bed for them, but if you've had the week of rain that I've had then I wouldn't worry about it. I can't even dig a hole right now because everything is so wet.

If the hole is big enough there is no problems. I sometimes plant 3 bareroot plants in the same hole that with about 10 inch between the individual plants. This way I get very impressive rosebushes that looks like one single big rosebush.
So I would not worry. I would sure use the holes you ahve dug. Also planting the clematis on the other side of the trellis is a good idea.
I do worry about these roses hardiness like athenainwi pointed out. But with really good winter protection they migh have a fighting chance. Good luck with your rose. And keep asking questions. I am sure people can help yo here.

Niels