There are two ways we recommend highlighting that an easement is co-held with one or more other organizations.
Note: Unless you treat those interests in a way that is different from your normal easements (most organizations don't) we do not recommend adding 'Co-held Easement' as its own property interest type.
1) Add 'Co-held' as a Property category
If you add 'Co-held Easement' to the list of available categories (via Settings --> List Items --> Properties) you can then easily filter out all co-held easements on any views or lists you're creating.
2) Add the Co-holder as an Active Contact
It is good practice to add a contact record for the co-holder as an Active Contact, and to tag them as a co-holder. You can then capture any details regarding reporting requirements, monitoring responsibilities, etc., in the parcel contact record.