After some thought, I don't think I had justification for concluding that there are two neutrino emissions. After all, we only selected for the emission of one lepton. I think there would be 2 leptons if there were two neutrinos (each W boson decaying into 1 lepton and 1 neutrino). I'm guessing it's likely that there's only one neutrino, which means I need to think more deeply about the missing transverse energy.