Ive tried and dont personally like the back/bi, chest/tri routine, but it all depends on what your goals are. My personal opinion is that, if you finish a chest or back workout and still have enough energy to do tris or bis, you didnt didnt train hard enough. The only thing i combine in the same day are bi/tri. Its like a leg day, if you can even think about doing another muscle group, you better go back and squat some more. Sure i do cheat somedays and dont follow this religously but its all in theory. Of course it depends on your goals and what youre training for too.