As for me, I tried playing with 2 druids: I only built one as a mage, but once I leveled Nature Magic to level 3 on the other, I slowly started pumping Tactics, and there are several reasons for this.
1. The Warden title gives a 10% defense bonus.
2. Why specifically a second druid hero? So as not to distract the main mage from summoning creatures and to cast such useful spells as Hive of Wasps, single or global Luck, First Strike, or Anti-Magic. If you summon a Water Elemental, you can drown the enemy in Quicksand together with the second mage, which is especially good against Life since only their angel flies while all others are quite slow; also, Life has no attacking spells and their archers are weaker than the elves', so you can shoot down the opponent from afar...