Oooooh I like this, what with adding supplemental air bags to a few of our vehicles. I'd never thought of plumbing a gauge in. Re-pressurizing was lots of manipulation between air hose and handheld gauge, your device could be part of the vehicle as well.
Most I've done are a single fill port, but added to the 'manifold' a needle valve to isolate right from left after filling. In theory it lessens body sway of that side under pressure in a turn, transferring to the opposite air bag. Haven't designed a test to prove hypothesis, mere seat of the pants testing says it works.
In one case, I didn't branch the supply lines to fill from one port, do to unsatisfactory solutions protecting the lines. Put those Schrader valves in web of lower rocker pinch welds.