Biceps and triceps can be worked at the same time. In fitness theory, there is a training method called antagonist muscle counter training, which is that two muscles with different mechanisms of action can be put together to exercise. While working one muscle, its antagonist muscle is properly relaxed to strengthen that muscle. And while exercising the other muscle, the strength of that muscle is restored. Exercising both antagonists together enhances the effect of the workout. Among them, biceps and triceps are a pair of antagonist muscles, so it is perfectly fine to workout biceps and triceps together.