How do you add another hand?

I assume what you actually want to add is a "staff" - the five parallel lines on which you notate music, whether it is intended to be used for one hand of the piano or for the voice or a flute or anything else. To add a staff, go to Edit / Instruments. If you want the staff to be part of an existing instrument - eg, two staves for the piano - then select the existing instruments and press Add Staff. If you want to add another instrument - like a voice, or a flute - then select it from the list at left.