The second passenger would have to buy a coach ticket. Then have an Amtrak agent cross-reference your two reservations to show that the "coach" passenger is actually in your room. I've done this before, and it actually produces for the second passenger a ticket that generically says "SUPERLINER ROOMETTE" instead of the usual room and car number. But the passenger's name will be next to your on the crew's passenger manifest. Without at least a notation in the reservation, you might have a hard time with meals in the diner, as Amtrak now prohibits sleeper passengers from making new friends in coach and putting them on their meal tabs.

I hope your voucher from AS is still unexpired, as I know the partnership with Amtrak ended last July. Best of luck!