Our international team is planning to use their own instance of Marketo. Can a workspace within Marketo connect to that instance of SalesForce?
Unfortunately, no. Marketo has a 1:1 relationship with SFDC, so one Marketo instance syncs with one SFDC instance. There was a previous question about it as well:One Marketo instance, Two Salesforce instances?
Devraj is right, but on the other end, the other way arround (1 SFDC Instance, 2 marketo instances) would be possible yet limited:the sync would work, but you could only have MSI with 1 Marketo instance. And it would be very trickty to set up...
Unless they also have a separate SFDC Org, I would strongly urge you to use one SFDC and one Marketo with Workspaces/Partitions.