Since the upstairs is over another living space, it sounds like your desire is to drive as much of the heat from the upstairs radiant floor UP through the floor and as little down through the ceiling as possible.
This will rely on getting as much insulation as you can up into the ceiling/floor joists.
In your situation, I suggest a couple of inches of rigid foam, if you have the room, placed between the I joists above lower part of the 'I'. Assuming 16" on center and a 3/8" I joist web,
- rib the insulation to 15 1/2",
- slip it up between the joists,
- rest it on the bottom chord of the joists and,
- if possible, run a bead of foam along all edges and seams to create an air barrier between the bottom of the foam and the space between the foam and the drywall below.
This would work fairly well to keep the lower floor ceiling from getting too much of the upper floor heat. This will cost well over $1000 in materials for 2" rigid insulation.
If you have the room, a less expensive and less effective alternative is as much batt insulation as you can fit in.