Ok Andrei, I may have to ruin 'em all. And David, I now have it working with both domains with site1.com as the primary.
We do want to make site2.com as the new primary name and depreciate the site1.com name over time. I was hoping to avoid it but you guys confirmed that my cookies must crumble!
My latest thinking is to do something like a blast email to everyone with username email and password so everyone will have it and can get the new *improved* site2.com cookie.