I have an army and it can use Feigned Fight.
I take an ally and they can use Feigned Fight.
Am a allowed to have one or two units with feigned fight ?
One. The wording seems clear enough to me. "An army with feigned flight can give the characteristic to a single ...".
The fact that the ally has the ability, allows you to choose an allied UG if you wish. If the ally didn't have the ability, you would be restricted to selecting a main army UG.
Richard