I used to have a feed sign/board up at the sliding door thingy to my horse's feed bucket. it had a thingy that one turned over, for am and pm. sort of like the magnet that some people have on their dishwasher, that says if the dishes are dirty or clean. anyways, that way, they wouldn't get fed twice by accident.
my friends had about 8 horses, and they would make the morning feed for all of them (all of the dry stuff anyways) the night before and they used different color buckets for am and pm. so, when they fed in the pm, you'd know, because the red am buckets would be filled and ready to go.
my other friend used a brick with am fed and pm fed painted on it.
she'd lay it on top of the feed bag .
good luck with your horses, i hope they'll be alright.
i would keep walking a little bit, keep them out and about, and call my vet, as well, just in case, at least as a head's up. and i'd watch them closely for a few days.