I think this is a great idea. Much of my garden is grown in this style. I usually plant the roses between 1 and 10 feet apart, depending on how aggressive they are. Since your rambler is already established, I'd go around 5 feet, measuring from the center of it, just to play it safe. Some of my results have been that the smaller rose just weaves through the rambler. Sometimes, it grows much taller, almost as if the two are competing against each other. If the rambler covers the roots of the other rose it can keep it cool during drought, no science here, just observation. Once or twice I got carried away and the roses pulled down the fence!I hope you enjoy the results. Regina

Okay.
Here are the details:
I have the spring-blooming rambler, Ispahan, growing on each side of the arch (a metal Cattle Panel bent into a hoop). The canes are very slender, but the clump is very dense. It blooms from 4-6 weeks every spring and then puts on lush green foliage. I planted a blue-violet Clematis at the inside base on the southside two years ago. Last year it bloomed although it wasn't very tall yet.
I want to plant the Climbing Pink Don Juan to grow up the same arbor. I think there is room to plant the Climbing rose on either side as the panel is 4 ft. wide. The hoop at peak is about 8 ft. tall or higher. Climbing Don Juan doesn't get the heavy canes as say, the New Dawn climber, and the Ispahan is very lightweight, so I think the panel will be sufficient to support them.

Thanks Regina.
I'm going for it! :)