右手と左手、同じ音からスタートだよ！って英語でなんて言うの？

If you play the keyboard note 'C' with your right index finger; and another 'C' in a lower key with your left index finger, then you will playing the note of 'C' in two positions with left and right hands. If you play the chord of 'C with left and right hands, this will require more fingers.

"Both hands start on the same note/key." This explains to the person that when you start both your right and left hands start by playing the same note/key on the piano. "Both your left and right hand start by playing the same note." This is another way of explaining that both of your hand start in the same place on either side of the piano.

By saying "both of your hands" you are including both right and left hands, so your listener will know that they should both start on the same note.

When playing the note or key or chord the same on both hands. "play the C, with both hands" or "can you find the same note, and play it using both the left and the right hand".

Piano 'keys', are the white or black blocks that you press with your fingers on the piano keyboard to make a note sound. Black keys were traditionally made of ebony, and the white keys were covered with strips of ivory but nowadays they are made of synthetic and usually sustainable substitutes.

When talking about music we refers to the sounds by using notes, key or chords (a/b/c etc) so if they 2 things do the same then you would say both to describe this

"Both hands begin..." is a more concise way to explain this instruction, while conveying it clearly. "Each hand should....at the start of the piece" is a more elaborated way to state this and would be appropriate if the person you are speaking to requires more clarification! For a non-piano player this might be a tricky thing to explain!