Two characters controlled by one controller in online matches

I didn’t download your project so what I say here might not apply, but here is how I would solve it:

Don’t try to make the controller control the actors by using build in mechanics but simply pass data from the controller to your actors.
Put one actor in variable A and the other in variable B.
And then in the controller blueprint:
Even left mouse button → get A → Shoot (event defined inside actor A) → get B → Shoot (event defined inside actor A).

Hope it helps.