So as you bring the gun back to your retention index, tug on the trigger guard with your support hand at the same time as your thumb hits the button as your support hand is leaving the pistol.
Wait, so you take your left/weak hand, move it up the pistol to be able to grab some trigger guard (and hopefully not overreach and grab the trigger or the index finger of the right/strong hand), and then go for the spare magazine? Despite the fact that the left/weak hand getting the spare mag and returning it to the gun is the limiting factor on reload speed?
I've never heard a competitive shooter advocate this technique or seen it done.