Nothing, and I repeat nothing needs to be done for at least 2 years. That means the 2013 season and the 2014 season. RG3 is on the books for about $6 mil a year and Cousins is on the books for under $1 mil!
That is, one QB that appears to be a franchise QB and a back-up that appears competent. If RG3 is as tender as you believe, 2 more years will paint a better picture and if he is ready for the rigors of the NFL game you will know in 2 years and it will give a couple years to showcase Cousins some if you expect to trade him.
No rush...and a great problem to have.