We have a small lounge with mullion windows. My preference would be roman blinds on both windows, but on one window it would virtually hide the mullions and curtains would work better. Curtains wouldn't work on the other window. Is it ok to mix?
My opinion is that it can look quite good, as long as there is a connection between the curtains and the blinds - for instance, using the curtain fabric to create a pelmet across the top of the Roman Blind for instance.
If you're using the same fabric for both, then there's no issue at all.