How can I add a second light to a circuit?
You can hook another recessed light in parallel with the first light. You can either run another cable from the junction box to the new light, or from the first light to the new light. 

There shouldn't be a problem using 12/2 if that's what you have on hand, but 14/2 is typically used and is cheaper.  

Remember to turn the power off, and verify that it is indeed off before beginning work.  And as always, if you don't feel comfortable doing the work, don't have the proper tools, or are just unsure of what you're doing. Call an electrician.