You have two Marketo instances that use the same email domain in the From line of their emails - "@example.com." Each instance generates it's own DKIM key for "m1._domainkey.example.com." However, because your DNS will only accept one key value for "m1._domainkey.example.com", you are not able to get DKIM working for both domains.
To resolve this issue, make sure that DKIM is set up and verified on one of the instances, and then open a support case requesting that the DKIM key be copied from the verified instance to your other Marketo instances. Once the copy process has been completed, both instance should use the same key, eliminating the conflict and allowing DKIM to be verified on both instances