How do you split an army into two or more separate armies?

What game?

You would need to put them into sleevies.

Perhaps this is the answer you seek, mysterious traveler.

I suggest that you avoid using the "Size Matters" module anyway, since the AI doesn't know how to use it properly.

Besides, Caveman2Cosmos already has plenty of bloat.

I'm assuming you're talking about HoMM V here (according to your account, that seems to be the last game you played):

Hold down SHIFT, left click the mouse on the army you want to split and then drag it to the new spot.

As a general rule of thumb:
if you ask a question specific to a certain game, it's well advised to name that game in your post.
This helps to avoid unnecessary confusion, and it makes it easier for others to help you. :)

Foment discord, spread rumours about the king's lineage, and encourage the discontent of the serfs until there is outright rebellion.

With a Swiss Army knife. Oh, wait, that's only for Swiss armies.

But he wants two armies, not a band of mercenaries.

The correct answer is to ask the general to promote his second-in-command in a touching ceremony that recognizes the many years serving their country, and assign part of the soldiers to the newly appointed general. Then you conscript more soldiers because otherwise the general will dislike having his army strength being cut to make a new army for someone who used to be under him.

Have them all raise their hands. Left handed people go to the left and right handed people go to the right.

Start a debate on the correct way to hang toilet paper

Fixed. ;)

To be fair, most of the game specific subforums are "dead" most of the time.
So I get why anybody rather posts their questions in the General Discussions (first).

But this becomes a self-perpetuating cycle -- dead forums will stay dead, lightly used ones will die, and some of the heavily visited ones might even become lightly used.
I would always go the opposite route -- post in the game forum, and if there're no (helpful) responses, or the problem is unresolved after a while, then (and only then) post in "General Discussion". (Also, how hard is it to just check to see if a particular game forum is a ghost town or not before deciding where to post?) There's already enough going on in General without adding game-specific topics (for games that are sold here and have very active dedicated forums) into the mix. This is not even taking into account that it will be much more likely for future visitors to be able to find threads with helpful advice for a particular game if those threads are actually in the right forum. I know I've found plenty of topics with useful info just by browsing through the forum for a particular game. But if such a topic were posted in GD, then unless it's been recently active on a day when I happen to be scanning the first page or two of threads regularly, or somebody directly links to it in another thread I happen to read, I'll likely never see it.

I agree basically with all that you've written, but how long is "after a while"?
After what time is it ok to post your question in the GD, especially if (in most cases) it's a question related to a game that you want to play right now - but can't (or can't to the full extend you wish you could - as in this case) because that question hasn't been answered yet?

Besides: to address your concerns regarding "future visitors of the games' specific subforums":
nobody is keeping you from posting the problem AND the solution to it, in the subforums.

As a matter of fact, just to prove my point, I made it just now:
https://www.gog.com/forum/heroes_of_might_and_magic_series/how_to_split_the_armies_in_homm_games_a_solution

:)