Posible options in my mind:
a. The tokens enter at the same time, but the rally ability only counts there being two creatures on at once: The Ondu Cleric, and the token who activated Ondu Cleric's ability. This happens twice, so 2*2 is 4. (Didn't seem likely, but hey, this might be the case)
b. The tokens enter one at a time, and the rally ability gives 2 health, then 3 health for a total of 5.
c. The tokens enter at the same time, and the rally ability counts 3 creatures both times it is activated for a total of 6 (2*3).
Optional Question: What if I started with 2 Ondu Clerics on the battlefield? Would that just double the health gained, or would something else happen?