Hello abadplanner
This is an intentional behaviour (by design). Currently, it is not possible to link the SBB Mobile App with multiple customer accounts. As a result, when we ticket inspectors scan the ticket, we can only see the Half Fare Card of the person who is using the SwissPass Mobile and not from any co-travellers. I cannot tell you if this will be changed in the future.
Kindly note, even though you can show your Half Fare Card through SwissPass Mobile (SBB Mobile App), you are required to have either your red SwissPass card or your passport / national ID card physically with you; ticket inspectors can ask for it at any time. See: swisspass.ch