Minor Credit Card Usage, AU or Primary?

[BronzeTrader]

I'm asking for a friend. He has  a 19 year old son. The son has his own credit cards and 6 months of active usage. But he likes his son to use his Chase Freedom (as AU) for UR points. The son can't get the Chase Freedom because of his low CL. 

My concern is that, as an AU, the son may not be able to build up his credit. I'm not totally sure since he does have his own, but would be turned into inactive usage.

[Gmood1]

He'll be fine. Just use the other cards for small purchases here and there.
My step daughter does something similar. She has 3 of her own now also 19. She prefers to use my Amex BCP that she's an AU on. All of her scores are mid to upper 700s now with 11 months of credit history.