Yep, have to agree a air valve is the way to go.
They can be bought very cheaply, a few bucks at most. They come in all sorts of sizes from 1 way to 8 way or more. Usually made of plastic or metal. There are various designs of these from a simple valve to one that you can hang on the back or side of the fish tank. Worth browsing through to see which you like best.
SImply put, they are a valve in which you attach a airline hose from the pump to the connector, and connect two separate airlines for each airstone.
Turn the valve handle to get the desired amount of bubbles coming out of airstones, cannot be simpler really.
But I will add one other thing, it is good practice to get a couple of non return or check valves as well. Again, these can be bought very cheaply, for a few bucks at most really. Comes in either plastic or metal.
This is basically to stop any water getting from the tank to your air pump which could irreversibly damage the pump (or worse!)
A simple diagram set up for a single airstone with a non return valve to give you the general idea. (not my pic btw, taken from a general online site)
And one last thing, ensure your air pump is powerful enough for 2 airstones. Some of these cheap airpumps are pretty weak powered that can only supply enough air for one airstone.
Though has to be said most half decent air pumps can power 2 or 3 airstone without too much trouble at all.