Online signing is available for co-landlords. The landlord creating the lease in their account can add the co-landlord when adding all parties to the lease. When finishing your lease, you will "request signatures" of your renters and your co-landlord. The renters will need to complete their signatures first, then your co-landlord will be notified via email to sign the lease.
Once all renters and the co-landlord have signed, the original landlord who sent the lease will need to sign into their Zillow Rental Manager account and ratify the lease by countersigning through their account.