Mixing decals and logical interaction

In my project I want to create area effects like freezing, seting on fire, electrifying, etc. The first idea was creating custom decals for every effect. The problem is in mixing them together. For example, ice + fire => puddle. So the formulas are part of logic, not just visual effect.

A is for ice, B is for fire. A&B is for puddle. No idea how to cut part of both decals and create there new decal. I can create grid of small triggerboxes covering every decal and create new decal where certain boxes are collided. But how can I achieve that? Can I combine meshes in decals? Any other ideas?