Can two non-similar instruments play the same thing at the same time?

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.