While (for example) your Ghost is summonned, you CAN summon (for example) the Undead !
You CAN, but if you DO, the already summonned (in this case the Ghost) will disappear. wink

That's why it's good to have that Crystal Skull as fast as possible (last time I was around level 12 when I had my Creature working for the first time), so you can, like Raze said, have 2 allies to help you smile
But if you like more allies, go for the skill Charm ! It's so fun !! laugh

On 7th of february 2015 : I start a new adventure in the Divinity world of Original Sin,
it's a Fantastic Freaking Fabulous Funny ... it's my All Time Favorite One !