I have any question about this card. It doesn't seem to have an option for a joint account (but does have authorized users).
It appears that only one Amtrak guest rewards number is tied to each card.

But if I sign up for the card and have my wife be an authorized user, can I have CO or United miles be deposited into her CO/United frequent flyer accounts for some months and into my CO/United frequent flyer accounts for other months? Or is it the case too that each Amtrak Guest rewards number can only tie into one CO and one United FF account?

The reasons we don't want to get separate MC accounts is that if we combine our spending on one card, we can more easily meet the $5,000 threshold for transferring to FF accounts.

Thanks.