How to mix radiant in slab w/ radiant in ceiling?

Our Eichler currently has radiant valence type heating in the ceiling. We are adding a new addition in which we want to put radiant in the slab. The issue is that the water in the slab needs to be a lower temperature than that in the ceiling, making it very difficult to run both off the same boiler. Has anyone faced this problem and come up with a creative solution?