You will see some variation in the sign for BOTH.

A rather formal version of the sign "both" forms the dominant hand into a palm-back "V" handshape, places it inside the non-dominant horizontal palm-back "C" hand. Then the dominant hand moves downward and changes into a "U" hand while the nondominant hand closes a bit into a "C" hand or an "S" hand.

Keep in mind though that when signed at high speed the sign for "both" may change quite a bit and the nondominant hand might move upward a bit (or a lot depending on the enthusiasm or intensity of the signer). 
 

both:

Notes: