Integrating Gainsight and Marketo when connected to two different SFDCs

Hi,

Looking for suggestions or ideas on how to integrate Gainsight with a Marketo instance. Both are connected to different SFDC instances that don't talk to each other. However, there is a need to sync things like unsubscribes and data updates between the two systems. 

I have looked into Zapier but that seems limited.

Has anybody encountered this before that might have a third party solution? Or is there some custom development we could look at to get these two to talk?