I think your concern about movement of the second barrel's bullet is valid.

I'd load both barrels, but only cap one. Then shoot the capped barrel and check the second load with the ramrod to see if there was any movement. If you do that the first two or three times you shoot a particular load and get no movement, you can be pretty confident of that load.

I'd do this any time I changed to a newbullet, sabot,or powder charge.