I'll admit I'm very new on managing more than one relationship at once, but I can't seem to see one of my partners as a secondary.
I've read definitions that say a Primary is someone who you share your house/finances/day-to-day situations, but I really fell like I can't just put people on primary/secondary classifications.
My way of seeing this is very similar to what Marcus posted.
I would hate if one of my partners had a date with me and put it aside for someone else, for example. I'd never do it to any of them, too.
What I'd do is explaining to the "primary" how I had said to the "secondary" I'd be with him on said time, and suggesting some other time for us to be togheter.
Much like I would do if I had an evening planned with friends and some other friend asked me out.