After extensive playing with a "multi-pet" playstyle, I have concluded some unfortunate news

The game seems hardwired to only support two active summons at a time. I will clarify further...
Sure, you can have 2 conuration minions, your merc, your dog, your horse if outdoors, etc, adding to 4+ minions out at a time, but...
If you watch closely, you will generally only see two of your minions are actively "attacking"
Storm thrall for example, is usually a ranged unit. If I also have a melee merc, and say... a daedric minion out, I will often see those two melee'ing the enemy, while the Storm Thrall is just tucked away in the back doing nothing. Then the next fight he might contribute, while one of the melee minions take a timeout. It's really silly and annoying.