Yes, the latest App versions allow this, and yes you do need to be on the same PNR, you can't add a separate one in.

However I would advise you to get the App on to her phone, it doesn't take long to install. For two reasons: the use of 2 BPs on one phone is a bit clunky at the gate and security (where relevant). You have to keep flicking between screens and of course it's always then when something happens to make a slick process grind to a halt. Secondly, if there is an irrop it gets horrifically complex trying to sync everything on 2 boarding passes (it's not that easy at the best of times, frankly). I also think the App is quite intuitive and so I can't think of a good reason not to have it.