This "trick" might not work on every inertia trigger system but it does work on mine.
You simply pull the safety catch back to safe poition and then slip it straight back to fire and bingo, pull the trigger and fire the second barrel.

Actually it only takes a fraction of a second longer to do than it takes to shift your finger to a second trigger on a two trigger gun!

The single trigger system is not as much of a problem as many people think.
It is actually a myth that inertia trigger systems require recoil to cock the tumbler for the second barrel.
Inertia trigger systems (that I've used) cock both barrels when you break the gun open just the same way as a two trigger gun or a single mechanical trigger gun does!