Hanging cafe curtain on 70 inch long window?

I'm attempting to hang heavy cafe curtains on a double window that is a total of 70 inches long. I really want to do an outside mount. There is not wood in the middle seperating the 2 windows ... just PVC. The company I'm using to make the drapes does not want to do it/says it can't/should not be done... they don't want to put a center support into the PVC. What do you think? Can it be done? If not, any suggestions on what kind of curtain rod I should use that might not require a center support?