2 eccentrics - I think!
What kind of rear dropouts have you ordered? With Rohloff's adjustable dropouts, I'm not sure you need 2 eccentrics, but only a front eccentric for the front belt.
I have never tried Rohloffs + belt drive, but I think that the combination of adjustable dropouts + front eccentric must be easier to deal with than 2 eccentrics, because
with 2 eccentrics, adjusting the rear eccentrics will increase tension on one belt, and loosen the other belt at the same time.