If you are working with a company that has multiple Marketo instances that require DKIM's on the same DNS, there are a few steps you need to follow and a few errors that can occur if you do not set this up correctly.
If you setup 2 DKIM records with the same selectors, it will cause both your DKIM's to fail so make sure you are using different selectors when setting up the record. The selector in your record is m1.
When setting up the record in your DNS you want to change the M1 to m2 as shown below (or m3, m4, etc if you have more than 2 instances on a DNS)
m1._domainkey.record2.com to m2._domainkey.record2.com so in your DNS you would have two records with different selector values
You then need to submit a Marketo support ticket asking them to add the m2 selector your to your instance, once its been added it's ready for testing!