Hypothetically, one can participate in as many religions as is practical and non-contradictory. Most religions are traditions, paths, not exclusive structures. Most religions in the world are polytheistic or animistic, and don't inherently preclude the existence of other gods--and consequently are capable of at least tolerance towards other practices, customs, and traditions. We are, in our Western mindset so framed by Christianity, trained to view other groups of religions as monolithic, when they are not. As an example, ancient Hellenic and Hellenistic religion is probably more accurately described as a patchwork of dozens of different Greek religions, plural, due to the differences from place to place--remember, the poets' vision of the gods did not necessarily match the popular belief and practice.