So what if we say we have two monsters on space 1 and two overgrowths in play. Does one overgrowth trigger on one monster and the other overgrowth on the other or are they just redundant stacks where it only ever works on one valid target and the other copies are just there to continue the effect in case other overgrowths are removed?
If there are two targets and two Overgrowths in play, the Overgrowths will advance separate targets. It's pretty bad news.
If there's just one Overgrowth you can pick which monster gets an extra move, though.