I see absolutely no reason why you can't feed then ride. I've been doing it for years with no ill effects. I've even *gasp* interrupted a horse in the middle of a meal to take them for a ride
Most cowboys I know who ride in feedlots will catch their horse in the dark in the morning, feed them a bucket of grain while they are being saddled, and then ride right into the day's work. I have yet to hear of any horse getting colic or having issues with that.
The only spot you might run into problems is if the horse gets dehydrated during your ride. If that happens, you might want to limit his feed after
the ride until he's taken a nice long drink. Making sure he's got some water in his gut will help to prevent an impaction.
If they are being fed in the same area but you are only taking one horse out for a ride, then catch the other and tie him/her up while you ride the first (so that he doesn't get all the feed eaten while you've got the other out). If they have separate feeding areas, then you've got nothing at all to worry about.