Give duo a cooldown, so that it can only be used 1 out of every 2 hours (or whatever). I mean conjies are sooo easy to play they have to have giant cooldowns, so why not muters? It would have the added benefit of making duo be used either offensively or defensively, since you couldn't both get the jump on people with it, and escape with it in the same fight.

Allow people with detect invis to see a muter X% of the time, and allow them to attack and bring them out of duo.

Chamo and duo being mutually incompatible makes very little IC sense, and is really a matter of 2-4 rounds of "visibility" over pretty long periods of time...

I don't know if muter can eat/drink while duo, but there is probably something like with assassins where manipulating stuff causes you to come out.

I should also probably play a muter, but I'm not sure what I'd learn other than I'm not patient enough to cherry pick this many kills... Mostly because it is boring as ####.