Back in my Warhammer Fantasy days, there were a few units that could reroll failed attacks or wounds, and a few that forced successful attacks or wounds to be rerolled. When they faced each other, you ended up with the hilarious sight of rolling every die twice, since you had to reroll all hits and misses and no roll could be rerolled more than once. In practice, a lot of players I knew just rolled once since it didn't matter.
For sanity's sake, I'd just say any completely opposite effects just don't do anything.