The second hero is also used because, for the reason stated by Qwerty:

4. Morale among troops of two alignments (assuming no undead) is neutral;

For example, it wouldn't hurt to take 40 gremlins or troglodytes into the army, just as cannon fodder, and 5 cyclops can provide good support. And if Shakti comes with 100 troglodytes on the right, you can level him up too! But on the other hand, for example, a Straker with 4 zombies is useless! But if the hero has +350 gold, you should buy something no matter what!
You need to be a little more careful when playing as Swamp, Tower, and Stronghold, because enemy troops give a movement penalty!