You have a couple of options to try. The quickest is to open the PHD2 Connect Equipment dialog, then click on the button with the 'branch' icon to the right of the drop-down list for cameras. That will show a list of the available ZWO cameras. Hopefully, the two instances of the ASI220MM Mini camera will have different device ids. You can do the same thing using the new-profile-wizard, which will automatically show you the choices as soon as you choose ZWO ASI as the camera type. If the two different cameras are indistinguishable, you could try connecting one using the ZWO ASCOM driver and see if the cameras can be disambiguated there. Beyond that, I can only suggest that you leave the main camera powered off, connect PHD2 to the external guide camera, then power up the main camera and let NINA connect to that.
Bruce