I just put in a plastic mesh divider during feeding time Doesn't seem to bother the fishies they get their fill i take out the divider then they swim off to where ever they want to lol.
Or if im feeling sort of lazy I Put in betta pellets wait for my betta to go for them then toss in the shrimp pellets on the other side of the tank for the cories really hard so they sink instantly and the betta gets to distracted to know what to do by that time the cories are already muching on some shrimp pellets