Can one person hold multiple religious beliefs?
It depends on what counts as a "religious belief" I guess.
If a religion believes in multiple gods I suppose in theory someone could mix and match. One might not believe in any gods but say an occasional prayer to one or more deities as an insurance policy of sorts.
Where beliefs relate to practises it's easy to mix and match. One might take OT law and avoid pork and shellfish, throw in a bit of Hinduism and avoid eating beef, but not actively practise anything recognisable as a religion outside of that.
It's hard to see how someone could combine the Christian God's insistence that he is the ony god with any form of recognition of other gods, although some might argue that a vegetarian Christian was combining teachings from the Hare Krishnas even if the vegetarian Christian was avoiding meat for health reasons rather than anything related to the Hare Krishnas.