Improved Overrun says, .... you receive a +2 bonus on checks made to overrun a foe(singular)
Greater Overrun, says, ... You receive a +2 bonus on checks to overrun a foe (singular). Whenever you overrun opponents (multiple), they provoke attacks of opportunity if they are knocked prone by your overrun.
... does it let you overrun multiple targets or only 1 target is knocked down but the other enemies adjacent have to "jump out of the way" who also provoke AoO.
It's simple grammar. You're good at doing X to a person. Whenever you do X to people, Y.