-I know that you need to somehow split the troops between two heroes and level them up at the same time (like, one goes to two sides) - but I can't figure it out. I always end up leveling up only one hero (because if you split the army into two, it turns out that it's difficult to kill strong neutrals with the main hero).

Honestly, I usually level up only one hero as well. However, I disagree that there won't be enough troops for the second hero: the very first captured city will fix this situation ;) And there is mentoring to get a lot of experience for the secondary hero.

Further, if the map is large, and your opponent is on the other side of the map, then if you go to him, it will take about a week or two - that is, his army will already have 1 or 2 weeks of growth more =\ (you can summon troops or something else, but if you captured a neutral castle along the way, then you won't be able to summon them anymore).

This is what secondary heroes are for: if there are many of them, the transfer of troops is very fast. And the enemy is unlikely to just sit around his home city; he probably also wants to expand his territory ;)

- I've heard a lot about hit-and-run tactics, and I've seen them myself. But I can't figure out how to do it (for example, with Warlocks) - do I need to level up mentoring and teach secondary heroes Armageddon?

Either that, or take a risk and directly level up the main hero and use hit-and-run tactics (the latter method is more relevant if you are playing as the Elves due to the Rangers).

- Is it worth attacking strong neutrals with losses (but getting experience for it) - or is it better to only attack those that you can 100% easily kill and move towards the opponent?

Experience is a good thing, but in my opinion, the decisive factor is not it, but what these strong neutrals are guarding. Also, it should be noted that strong and hard-to-kill are different concepts.

- How can mages attack dragons without taking losses =\?

If we are talking about mages, then obviously summoning is the answer. Various phantoms, elementals, and crystals with walls of swords are definitely not the worst weapon against them.

- Regarding tactics: is it worth sending a strong squad directly into the enemy ranks to inflict damage (but it will be killed immediately) - for example, djinns?

If they focus fire on it, then it is unlikely that the damage inflicted by this squad will exceed the damage received. Especially if we are talking about djinns: they are killed very quickly, and the unit is useful at least because of the spells.

- Regarding the division of low-level units into single units - if you do this, there will not be enough space for other units (let's say rakshasas, djinns, golems), is this justified?)

Dividing into single units is justified if
a) you simply have empty stacks (at the beginning of the game, they are naturally empty)
b) if you want to level up the second hero and have a non-zero army for him right away.
That's my position, but I would be glad if experienced players in online games would express their opinion.