I cannot understand the reason for this design decision. It seems to add needless complexity and discomfort for the player without being fun or making any sense.
First, the making sense part. Champions are people who become developmentally handicapped in the presence of other champions. They are smart as a whip sitting back and letting 6 squads dismantle the opposition, taking notes and learning the ways of uber-pwnage. But with two champions, what happens, is there only one pen and paper for the entire squad? Do they have to take turns writing and split up each other's notes afterwards?
I know this rule was instituted when it was discovered that champions were so powerful that you could beat the game with them without ever building units. This strategy offended those in power long ago, and since then champions were neutered with exp split, general exp decline, and spiced up with that just three to six crummy levels til I get the cool ability feeling. At the same time, units got an extreme buff, and now you can beat the game easily without ever using a champion (or having a champion be useful), but there is no outcry. What gives there?
Now there is the effect on the player. Players have to build an army for each champion, or resign themselves to just having fancy backstories to their unrest reduction in some city. The player must manage these multiple armies, which will never have enough map to level them all, all the while wondering, is this how Peter Venkman felt when Egon told him to never cross the streams? I mean you CAN use them together to win that hard battle (maybe against a giant marshmallow man), with all that juicy exp... which gets split up to the point its just one more stride on the long mile to level 10 or 12, or whatever level a champion actually will feel heroic at. Fellow champions are each other's kryptonite, which makes it a difficult strategic decision whether to use them together, but it's like a choice between crummy or crummier to the player.
This forces a player who knows the split exists to play with far more micromanagement and complexity in the hope, in my experience in vain anyway, that your heroes will eventually be, you know, heroic. I mean make it to the level ups that are fun (the ones that aren't +1-3 to a stat that doesn't make much difference). Players who don't know it exists will just wonder "why are the trees so long"?
I know this is a negative post, but hey, I strongly feel this is a bad decision through and through. The game will be better and more friendly and logical to every newbie, at the least. But I think it may even make people who are strategy diehards have fun teaming up heroes without having to worry about fighting 2-5 times as many battles to get where almost no champion but the sovereign gets in a normal game now. It makes sense and it is a fun, simple strategy to band champions together, and it is bad design to discourage logical, simple, fun gameplay. It is not unthinkable there was another way to encourage more complex gameplay without killing the fun rpg parts of the game or forcing players to juggle making and using many armies.
Btw, I think the game is great and should get deep and wide acclaim, but I think decisions like this endanger it to a possible dilution with "meh" because it doesn't pander to the most visceral and powerful source of fun in games with rpg aspects, the ego identification with heroes and the player's character. I have posted about this before, but basically every game that has ruled this genre has been at best a decent strategy game suped up with a fat layer of ego satisfaction. I think this is a great strategy game that has been drowning out its own ego attraction in the name of balance.