Sorry for the late reply, but it's taken me quite some time to finally figure this out, but voila!
1.) 36 inch balloons... they last forever, no problems.
2.) Get a hydroponic air pump: this one is perfect...
http://www.amazon.com/Dual-Diaphragm.../dp/B003AKRUSW
3.) 30 feet of small diameter clear hose, 15 feet for each balloon - connect to each balloon to the hose - I'm sure you can figure out a good way.
4.) Plug each 15-foot section into the pump, hit the button. Happy times.
Also, since you don't want to have to plug in and then unplug when you want to stop, go to a local hardware store and get a foot-activited extension cord, with a button that you push in order to get power. Then you've got something easy to turn on and off.
The pump referenced above gives just enough flow to last 3 to 5 minutes until they are too large. (no such thing, really...)
The problem with splitters, using only one air line, is that the air will sometimes flow from one balloon to the other, leaving one big, and potentially one small... the air will always want to equalize. So, by having the pump with separate attachment points, everything works perfectly, equal pressure from an independent source.
My wife thought it was clever.
Good luck...