Welcome to FT!

I think that the purpose of having an alliance is that when travelling internationally you can base all of your frequent flyer needs to a single airline in the alliance, irrespective of airline traveled. Thus, it is probably not recommended to have accounts with both AC and UA.

However, since you are already on AC, I'd recommend sticking with them, especially if you are well on your way to elite status. On a side note, I do not expect AC's award travel fee to stick... there will be way to much backlash.