Running multiple air stones from a single air pump depends on the power of it chances are, if tis got more then one outlet, it should be able to power more then 1 small air stone.
Well ive ordered one and should be with me thursday, was just concerned it would maybe effect the fish going from having one running to none at all, however my filter does produce quite a bubbly current so thats producing plenty of oxygen i imagine.
air stones may not oxygenate the water that much, but they provide needed circulation in large tanks.
i found this out from setting up a divider in my 30" long tank. without an airstone, the temperature is 74F on the side without the heater. with the airstone, the temperature is 77F on that side. not a big difference, but it might be in larger tanks.
Buy a wooden airblock from your LFS much better (finer bubbles) As said before, water surface aggitation is much more important for gas exchange.
Regards
BigC