In this instance, the second passenger should still be linked to the roomette, for two reasons. One, since Amtrak doesn't overbook this allows Amtrak to sell the actual coach seat to someone else. Two, by being associated with an accommodation the passenger is entitled to the coach fare associated with the cheapest fare bucket (for example, if a coach ticket sells for between $80 and $120 depending on how booked the train is, a roomette passenger is always entitled to pay $80 for the coach fare no matter what--although the roomette price can and does fluctuate depending on availability as well).