When waiting for an engagement to occur the loader wouldn't be standing around holding an extra round. It's too heavy and he might have to do something in the meantime that required the use of one, or both, of his hands. A loader, when "lap loading" would take out the appropriate second round only after hearing the fire command just so that the situation you describe wouldn't happen. Then, after loading the round into the breech, immediately get another round of the same type, if available, from the ready rack (or the first four to six spots of upright rack next to the main gun, making sure to stay out of the path of recoil). Lap loading can be done on the move provided the ground the tank is moving over is not too rough. Of course it helps if there are so many empty casings piled around the loader's feet that he can't move them (been there, done that).
For me the reloading process of the M60s is too slow. There seems to be, to me anyway, a very noticeable delay between the main gun firing and the loader starting to take a round out of a rack position.