Forms rule to invalidate field if contains value from other field person or group

I have two fields on a Nintex form: Document Owner and Document Approver(s). Both are person or group fields. 

I need to create a rule that if the Document Approver(s) field contains the same name as the Document Owner field, it becomes invalidated. The Document Approver(s) field is not restricted to only one person.

Essentially, I need to limit a person from being both an owner and approver for a document and would like to do this through rules on the Nintex Form. 

I can make it work with only one name in the Document Approver(s) field using this rule: DocOwner === DocApprovers, but that validates with equals, not contains.