It would make no sense to play one keeper in the league and one in CL. Copa is okay but to perform on the biggest stage you have to have as much playing time as possible to click with the defenders in front of you.
Real got very lucky this year that Casillas' mistake in the final did not lead to a defeat. And you could clearly see in the WC what he has become without enough playing time.
We need one definite no. 1 (and I hope it is going to be MAtS). You can of course switch keepers, even in the middle of the season, when the no. 1 under performs but then it has to be a clear cut and the keeper who has been on the bench must be the new starter.