meh. too much work. either call and get some pre-mixed coke in a corny, then mix some capt. in there, or just suck it up and buy some 2 liters.
non druck edit:
if you mix them right before the party, they should be good for a while. But then I think you might loose some carbonation. Just shut the tap off, recarb and go.
If you know someone who works at a restaurant, talk to the manager. See if you can buy 4 gallons of coke from them at a decent price.