Clarification on sharing a neutral for 2 circuits
You'll use a double pole 20 ampere breaker. It will have two handles that are tied together, and will say "20" on the handles. 

It should look similar to this...


(source: homedepot.com)  

I'm not sure why you're planning to run 10 AWG conductors, but typically 20 ampere circuits only require 12 AWG conductors.