I'm also looking for a way to integrate more than one SFDC with Marketo. It looks like a Middleware option is the way to go. Is anyone using this method currently? What Middleware option do you use?
I see that Josh mentioned Boomi, how does this tool work?
This is very interesting. Josh Hill have you worked with Boomi before/know someone that has?
I work with people who use Boomi and Talend. Best to ask an agency for assistance if you don't have that in house.