...will spawn per Mushroom island area when you combine this with Quark: Realistic world generation. If this is a Quark issue let me know and I will report there.

version 1.0.0
Forge 14.23.4.2439

This could just be you getting unlucky (the passive mob cap is only 10 mobs, and it fills up fast). In my test world I was able to get both spawning on one island.

But I'll look in to another way of spawning the mobs, maybe I can spawn them alongside red ones or something.

Can I get a seed and coordinates of the island, btw?