hi
usually not allowed. but depend on area its different.
the posh area gulshan; baridhara, banani- u can, mostly no one will bother you if you stay with your girl friend, but make sure when u r hiring the flat- your friend should be with you.
others area- actually if you say both you are couple--its enough. but if you declare that you are friend...no area in bangladesh- will give you flat!!!