I got the impression you were asking whether someone can practice different religions simultaneously. Let me share my thoughts on this. As you may know, I consider the existence of a variety of religions useful for humanity. In the preliminary stages of one's spiritual research, one can practice both Christianity and the Bhuddadharma. One can, for example, respect and have faith in the Buddha's teachings on non-violence, compassion, and tolerance, while remaining skeptical about karma and reincarnation, and basically believe in the Creator and feel close to God. At that level, I think it is possible to practice two or even more traditions. It is like being in school: as long as you remain at the general level, you may study a range of subjects. But as you progress to higher studies, you should choose one specialization.
For someone who goes deeper into Buddhist practice, which is based on voidness, interdependence, and no absolute, there is no place for belief in a creator. The opposite is also true. For a Christian, the essential points are the Creator, love of God, and love of fellow human beings. I asked a Christian priest and friend of mine why the theory of rebirth was unacceptable to a Christian of deep conviction. He replied, "The belief that this very life, without any other one preceeding it, is created by God is what develops the feeling of intimacy with the Creator." I saw a positive meaning in that. For a genuine Christian, it is not possible to accept rebirth and, even more important for us Buddhists, the belief that everything is interdependent. When you reach a certain level of practice, you have to make a choice.