DThe McCosker's Flasher Wrasse requires a 55 gallon aquarium (or larger) with a tight-fitting lid since it is a jumper. Often found along reef slopes and rubble zones, the McCosker's Flasher Wrasse will often swim near the bottom and tends to form small groups where a male swims back and forth between groups of females. The colors of an adult male intensifies during courtship and he may flare or "flash" his fins to attract females. We recommend only one male per aquarium display. DRS.FOSTER SMITH
IME keeping multiple male flashers is the way to go. Their color intensifies and they display throughout the day. I would go for 3 or more, as they set up a hierarchy. If you only got 2 one would chase the other. Multiple specimens keeps the dominant male busy displaying his dominance.
If you look through the threads about flashers you will find a lot of people keeping multiple males whether same species or different. I currently have a mckoskers, a lineopunctatus, and 2 flavianalis in the same tank and they are fine.