Your formula for the max(enemy1,enemy2) is incorrect. Given your example of 75% and 50%, if you miss the first one then you still have 50% chance of hitting the second one. You don't add them to get 125%, instead it is P += Pn * (1 - P) so you get .75 + .5*(1-.75) = .875
This is what I'm doing in Neuromancer. I guess it isn't exactly an XOR, but it does take both the hit probability and the miss probability into account correctly for multiple enemies.