SPG and PC permit this but not Marriott. You will not see the other name on the reservation. I suggest you contact the property to ascertain that the other name is there and note the individual and the day you spoke to them. Then, a day before your stay -- if you are really worried -- contact the hotel and see if the companion's name is still in the system.

I can't think of anyone who reported having place an additional name on a reservation via contacting Marriott or the hotel, having reported that this did not occur and their guest was in some way not permitted to check in.