Mon Mar 03, 2014 4:01 am
The way multiple stack participation is handled is complicated and Jim-NC (who is a very reliable source) addresses that aspect of your question.
As I understand it (and your question throws me for a loop because I find after doing some searches that I cannot say this with 100% certainty: I'm a less reliable source) stacks remain conceptually separate during combat. An element in combat takes its command bonuses (from leader ratings) and its command penalties (from CP penalties) at the stack level, so if one stack has a -10% CP penalty, all elements in that stack suffer the penalty. If a second stack enters the battle that has no command penalty, all of its elements will fight without any command penalties while all of the first stack still has -10%: they do not "merge" although they are all displayed together in the Battle Results screen.
In the same way, the most senior leader in the region does not matter, it is the senior leader in the stack that is responsible for leader bonuses (and the unit leaders too, but I'm simplifying). Army leaders pass some of their bonus to their Corps commanders but that is a separate mechanism, so if there were an Army and two Corps, they would still all be treated as separate stacks, with the Army commander's stats going to the elements in his Army stack, and the leaders of each Corps' stats going to elements in their own respective Corps stacks. If one of the stacks does not have enough CPs it will not affect the CP's in other stacks (and excess CPs in one stack do not get transferred to one that is short).
That being said, it is usually inadvisable to have multiple stacks trying to participate in combat for the reasons Jim-NC covers. I want to highlight something he mentioned but did not detail the importance of: the very high chance that one or more of your stacks will not participate, leaving a small stack to be overwhelmed and completely eliminated even though there are enough other troops in the region to successfully fight the battle. Experienced players in other threads (Jim-NC among them if I recall correctly) have advised accepting the CP penalty to guarantee that every available unit fights in the battle rather than trying to keep everyone fully commanded by dispersing into smaller stacks. You risk not just unwanted retreats, but outright destruction of the smaller stacks.
Exceptions to this are forces that you want to remain inside the local structure even if the outside defenders withdraw, and Army and Corps stacks that can March To the Sound of the Guns. Armies and subordinate Corps stacks have a very HIGH chance of participating in support of each other via MTSG if they are in the same region and are safe to leave uncombined in anticipation of combat.