Today, we use purposeful duplicates for this (see my reply here: Moving leads back through the revenue modeler ). We're in the process of re-architecting this to minimize the number of long-term duplicates and works from "single record of truth" approach.
BTW, as we're about to upgrade to 365 Online, we heard that Microsoft recently changed the authentication method for Microsoft Dynamics 365. As a result, Marketo’s integration user may fail logging into our customers’ CRMs. There is a P1 Jira ticket open for the engineering team who are working directly with Microsoft to resolve. No ETA yet. Are you experiencing this issue?