Yes, you can summon one creature with a spell and your necromantic creature at the same time. This way you can combine a lot of strategies. Just a couple of examples:
Ghost + Creature with melee skills: you and the creature engage in close combat while the ghosts makes ranged attacks and heals you.
Undead + Creature with ranged magic: both your summons blast enemies from afar, you do whatever you like in combination.
Demon + Creature with melee skills: you release your hounds while you send volleys of arrows from a comfortable distance.
Etc, etc, I'm sure you'll come up with other and more original strategies as you experiment with the different creature skills and summons you have.
