Questions about the gameplay of Heroes 6.

Could you please elaborate on this point?
1. Should both heroes have the training skill, or is it sufficient for only one of them to have it?
2. What does it mean that skill points are transferred? If one hero has 5 skill points and transfers them to another, does the first hero end up with 0?
3. Are skill points only transferred when the skill has not yet been learned, or does it not matter?

First of all, experience is transferred.
Secondly, they are not transferred, but added. There is Hero A, who has 10,000 experience points (not skill points!) and Hero B, who has 1,000 experience points ("exp" hereafter).
Hero A has a skill that allows him to train Hero B.
Hero B does not necessarily need to have this skill. As a result of the training, Hero B's experience becomes 7500, i.e., 75% of Hero A's experience. It becomes that amount immediately, and is not added to what he already has! (it cannot be reduced).

The new thing is that in Heroes 5, I could choose, for example, dark magic and learn a lot of spells, but here it turns out that there is only one level for one spell.

Well, that's true :) Although there weren't that many spells there either