In threads about the subject on the Muzzle Loading Forum, it's mentioned that one safe loading method for the 2 balls is once started ~1/2 inch apart, to then proceed to ram the two balls at the same time so that air will escape through the powder and the nipple as usually happens when loading only one ball.
Also mentioned is that some air probably escapes through the rifling grooves even when loading one patched ball at a time since no extra ramming resistance is usually encountered when doing it that way. If air was being trapped then someone would be able to notice extra resistance on the way down when the 2nd ball is being rammed.
If there was any question about the depth of the 2nd ball then the ramrod can be marked for the proper seating depth to check that the load is properly seated.
It's important that the balls are in contact on top of the powder when they're loaded. They will flatten out a little bit upon ignition where they are in contact throughout their travel through the bore together until exiting the barrel.
The accuracy of such loads have been shown to sometimes be outstanding.
It was also suggested to load with at least 20% less than the maximum amount of powder.
Recognizing that it's normally safe to shoot 2 ball loads when properly loaded is one reason why Remington makes the loads shown above.