Hello Thoissiewhourr57
I recommend making two separate purchases, as the Half Fare Travelcard PLUS can only be used to purchase tickets or day passes for the person to whom it is registered. Proceeding with the purchase for a third person may block the process. Additionally, payment for one cart with two different payment methods is currently only available on the website, SBB.ch, and not on the SBB Mobile app (and only if the amount on your Half Fare Travelcard PLUS is not sufficient to cover the total cost).
I wish you a nice day,
Chiara_S