First up, let's correctly understand the terminologies at play here, Person data is stored in Partitions (and not Workspaces), whereas marketing assets are housed in workspaces and they reference the person data from the partitions they have access to. Check out the product documentation on Marketo Workspaces and Partitions here. Additionally, a person can only exist in a single-person partition, you cannot share the same person record across multiple-person partitions. The workaround to this is to have duplicate person records in both the person partitions and keep them in sync, however, this is more difficult than it sounds when done at scale. If you want to implement custom sync rules, you can reach out to Marketo support to update your de-dupe rules so both the email address and person's partition id are taken into consideration for de-duplication.
Furthermore, when a person partition is shared between multiple workspaces, all the marketing assets (smart list, campaign, etc.) can access the person records in it, which is what you need I think (but let me know if not). I'd highly recommend you check the documentation on person partition and workspaces and understand its dynamics clearly before going into the rabbit hole of purposeful duplicates/setting custom deb-dupe rules.
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