I can see the appeal of smooth panning that you want with the fluid head, while still being able to level.Set the tripod up. Level with the ball head, then i can pan on the fluid head for the ultimate in smooth and stable glassing/ranging setup.
- Have you looked at leveling bases, instead of ball head + pan head?
- Or tripods with built-in leveling - Leofoto has some - but the ROM is obviously not as wide as a ball head. You can put any head you want on it though.
Downsides would be locking everything down tight (two heads), for shooting clamped.