Thanks for your answer Celludriel.
I totally agree with you: doubling the spawn would make the game incredibly hard.
And it's PRECISELY what I want!

Would force the player to use tactics (spell / environnement / hide in shadows / ...) even before the fight, to maximise his survival chances.
Would also force him to min max his characters, search for and craft the best weapons & armors possible, only to be able to barely survive.
I can't be alone finding this truely exciting, can't I?