Can you be a Buddhist and believe in another religion at the same time?
Actually no. One who has become a "Buddhist", meaning having entered the stream, having the Tripple Gems as firm refuge, does no more seek refuge anywhere else, does no more value other teachings then the Sublime Buddhas Dhamma.
But it's of course possible to make use of the Dhamma, the right practice for it, if one desires for becoming in this or them realm, desires for this or that re-union, even for good ways in this sensual realm. So even if one desires toward union with certain Brahma, God, , good wandering on, one is able to take on proper advices, by the Sublime Buddhas Dhamma, to go for it, good Brahman.