Maybe 6 adults? If it's barebottom you could do 6-8 juvies for growing out. What are you going to put on the other side? It may stress them out depending on the fish? I'd not divide it, get 8 adults, plant it, and sit back and stare at a great tank!
Both types of fish would do fine at 6.8 pH and a temp of 84 degrees. You may find however that keeping the nitrate level near 10 ppm for the discus will be a challenge with large FH in the same tank. Also keeping 2 FH in that small of an area will not likely work.
Personally you want to give discus the most room as possible...imagine if they got spooked they would go beserk in a cramped space. What is the reason for seperating them?