It cross contaminates your brush- mixing liquids can cause service breakdown-
Allen Rapley, NZ Education Ambassador for CND, said this a while back...
There is an accelerant in Moxie that when mixed with Retention+ would alter the reaction causing it to weaken the chemical bonding.
Seperate brushs is a good idea, but not a necessity. What I would also suggest is to prepare the brush in the new monomer before using. Put a tiny amount in dappen dish, saturate brush, wipe out, throw out remainder of monomer then refill with fresh liquid. this stops any chance of contamination.
Creative Nail Design (NZ) Educational Ambassador