Of course, you can dope with two or more dopant in CeO2 because the structure is quite open. I have attached one of my early paper "Effect of solid solution additives on the sintering of ultra-fine CeO2 powders", hope it is useful to you.
Sorry for very late response. I can find that you have got most of your responses. For doping a CeO2 material, we have to keep in mind the ionic radius of the dopant matching with that of Cerium. I really did not understand your first question.