You will have to use one system as the final word, which should be Marketo.
- Every week you should sync files between systems (most jurisdictions have a 10 day grace period)
- output all unsubscribes
- import to Marketo as Email Address | Unsubscribe
- export marketo's unsubscribe table with the values in #3
- import back into other systems
you can probably just do the most recent unsubs if your system allows you to do that easily.
If you can get someone to program the API, you can connect the systems directly and automate it all.