Is it necessary to have 2 heroes per castle?

You definitely need a second hero (example): You're playing as Gilraen, and at the start, you build a tavern (if you're starting from scratch). Suddenly, you see Ossir drinking in the tavern:))) Don't you like elf archers (do you?) So, you should definitely take him – that's the first thing. (Another case: if it's Dirael or Anwen, they'll offer less help, but they'll still offer some.) Secondly, you need at least one support hero for troop transportation. With the main hero, you clear out the mobs, and with the second, you run after them, collecting stones and wood. As soon as a week passes or a new unit becomes available, you rush back home with the support hero and transfer them to the first hero. Thirdly, these support heroes might have a tent (Ilfina), a ballista (Talanar), etc. Moreover, Talanar seems to have a skill for gathering troops.
I usually level up the support hero to at least level 3 or 5. And if you already have several towns, you take an army with the support hero and level up another one – the result is two armies with good heroes.
I always try to buy support heroes (depending on the difficulty):smile20: