If you add both an iPhone and an iPad to your Duo user (meaning, if you log into the Duo Admin Panel and view the user you see both devices attached to the user and activated for Duo Push), then that Duo cloud account will show up in Duo Mobile on both devices.
Third-party accounts, which are accounts that only produce OTP codes for login to other services, and which you set up on your phone by visiting that service, editing your login settings to turn on MFA with an authenticator app, and scanning a barcode provided by that service with Duo Mobile, will only show up on the device where they were added. Today there is no way to sync your Duo Mobile third-party OTP accounts between two devices.
