Homm5 Skill School

To be honest, I usually level up only one hero. However, I don't agree that there won't be enough troops for the second hero: capturing the first city will fix this situation. And there's mentorship to help the secondary hero gain a lot of experience.

It's not necessary to take troops from the second city. In most cases, the Wizard will be fine with stalkers and alt-grey witches, or even just stalkers. Machinists don't need many troops at all, especially human ones.
There are many other clever combinations you can come up with: through direct hero attacks, or through certain unit features. You don't need to use the entire army very often.