Would the bonus on flanking still be granted if it's not the characters main weapon that has the enchantment but a +1 menacing Barbazu Beard or Armor Spikes? If that's the case, menacing would be a pretty bad weapon to put on your weapon, unless it's only a +1 one.
Yes, you just to activate the effect you just need to be near a flanked opponent. Technically you could even put it on a ranged weapon, because you don't actually need to be flanking said opponent to grant the bonus.
In that case menacing is probably one of the worst enchantments to put on your weapon since it's much more efficient to simple wear +1 menacing armor spikes or barbazu beards if your main weapon is +2 or better.
Does several menacing weapons stack?
It might be ruled out, due to 'multiple similar effects', but works in a quite different way that normal spell effects.
The fact that meanicing has the effect that, "the flanking bonus on attack rolls for all flanking allies increases by +2" instead of just changing the flanking bonus to +4, could imply that they can be stacked on top of eachother.
EDIT: I wasn't clear from what I wrote, but I don't suggest one character with several menacing weapons, to cheesy out a great bonus. I was thinking several characters who each got a menacing weapon.
I'd say no. Maybe it's just worded that way to be used with Outflank.
Well that makes sense. I haven't really included the teamwork feats into my realm of consiousness yet.