Newbies will commonly separate armor (1 space back) from infantry due to the perception that inf is better on defence.
The reality of the game for veterans is that the biggest stack rules, so the better action is to place both together and everything else you got into the biggest stack you can make. The armor still has the 2 move reach for rapid deployment on your next turn.
A similar thing happens with bombers and fighters, but not as much.
yes the bigger stack rules but you can always avoid them. depends on whether you’re fighting a war of attrition. japan should realize it’s going to lose (in beginning) most of its land territory because it needs to consolidate–as you allude to–all of its forces on one territory. otherwise they would lose the units AND the territory.