It's a balance issue. Two cops could be a flood of information to the town. It'd be possible that the town could know both mafia members on Day 2, which isn't fair. Two jailkeepers would be less of a threat than two cops, probably, because Jailkeepers can also end up blocking town actions.
But the bigger problem is the fact that if someone claims to be a cop, there's less significance, because there may be two cops. I guess I could see that being interesting in its own way. Someone has probably brought that suggestion up at mafiscum.net, so I'll look into it.