He probably jumped, Bettas are jumpers! Or the divider had a hole in it or some sort, but he probably jumped.
A divided 2.5g is not recommended, I would get Cisco his own tank right away! Cisco is probably just scared from the encounter he ran into, or jumped into. He'll calm down in a few days or so, getting his own tank will help with that.
Lucky you didn't end up with badly beaten boys, some members here have been not so lucky when they're Bettas hopped across the divider. :P