Yes, it’s perfectly fine for two instruments to play the same thing. This is commonly called “doubling” in the orchestral world, and is commonly used for reasons such as:
- Dynamics – having multiple instruments play important figures in unison or at an octave will make those figures louder.
- Timbre/tone – by combining instruments it becomes possible to create new, interesting sounds. Composers often play with different instrument combinations to get the exact sound they want.
- Balance – one of the biggest challenges in writing for a large ensemble is finding ways to make sure all of the parts blend together in a homogenous fashion. Doubling helps to make the ensemble sound as one gigantic instrument rather than many distinct instruments.