Can a Buddhist mix Buddhism with other religions? Is there any authoritative sourced that a man can have more than one religion if he pleases?
It seems significant that HHDL advises us not to abandon the religion of our birth yet also encourages us to strive for realisation. This is the view that religion is a vehicle, not a destination. 

All those I know who endorse more than one religion also say, equivalently, that they have no religion. It depends whether we see our religion as a proscribed set of beliefs or a set of methods and practices for transcending belief. 

And then, talk to any three Christians and they will probably have three different religions, such is the game of Chinese Whispers.