Metal & Petricite Dragon (duo dragon pair that spawn together in the pit)
1: +8% Armor and MR
2: +13% Armor and MR
3: +18% Armor and MR
They would be considered the same unit despite separate health bars (the team that kills the remaining dragon is counted as the team who killed both). If one is left alive, the other goes and picks its friend up and they both go back to the pit.
Of course, there being two of them means they are particularly weak against AoE or multitarget attacks but their strength would be their ability to deal with someone trying to solo them.
Metal Dragon would deal physical damage and reduce armor by (1 + 0.5 per level) on each attack.
Petricite Dragon would deal magic damage and reduce magic resist by (1 + 0.25 per level) on each attack.
Both would be single target.
So focusing one over the other while solo'ing them results in the other dealing a lot more damage once you turn to fight it.
I think that's enough for uniqueness of these dragons, but you could go another route and give Metal higher armor with lower mr and vice versa for Petricite.
Their emblem could be some form of black and white symbol, possibly something akin to a Yin Yang.