In order to enable multi-signer on an application, you will first have to toggle enable multi-signer on in your application editor. And then follow a few additional steps from your mapped PDF to your application.
These instructions in the tooltip on the enable multi-signer button allows you to follow these steps to complete. Everything but the one additional step you would need to make on the application editor would be ensuring that you have the multiple owners section or field added on your application, which is found here under the key multiple. Owners.