I have a dominant gelding who is in with a pony mare, he is food aggressive and if I only have 1-2 piles the pony would never get to eat. Soooo... I take their hay ration, separate it out by flakes, and place each flake about 20 feet away from the next. He has gotten better with this method, so now I can have 2-3 flakes per pile, but I still usually have 6-7 little piles of hay around.
Just a thought, when I went on vacation I had 2 round bales delivered in order to make it easier for my relatives to take care of the horses. I had 2 because of the gelding's aggression, I didn't think he'd let the pony eat if I just had one.. Much to my surprise they ate off the same one constantly. There was no aggression on his part towards her..
I don't really like separating long term companions, but if you think it's the best for your situation make sure you have the new horse lined up to come over relatively quickly.