Duplex Rooms are not room for two. See the diagrams I have posted on the previous page, and the discussion pointed to in the same post.
Duplex rooms are vertically staggered single rooms of a slightly more luxurious proportions than Slumbercoach singles.
IIRC the full line of sleeper designation's, they were: Single Slumbercoach rooms, in 'standard' 24-8 type Slumbercoaches, and Double Slumbercoach rooms. In the 16-10 type Slumbercoaches, the Singles's 1,2,3, and 4 were in the rooms designed for two, but only had one bunk. The rest of the singles were of the "duplex" design, staggered up or down alternately. All of the singles's regardless of design, were sold at the same rate.
"First Class" or Pullman accommodations, started with Upper Berth's, then Lower Berth's, than full Section's (Upper and Lower sold together), then Duplex Roomettes (simiar to Single duplex Slumbercoaches but slightly larger and more plush, then Roomettes, than Duplex Single Rooms (with beds cross-wise to the car, but staggered up or down, followed by Double Bedrooms, Compartments, Drawing Rooms, Master Rooms, Bedroom Suites (Two Double Bedrooms), or Double Bedroom/Compartment combinations.
The Slumbercoaches could be purchased with a coach fare, and the room charge. The "First Class" accommodations required a First Class fare, plus the room charge.
There were also other fare plans for "Tourist Sleepers", usually in older First Class equipment, sold at coach fare plus room charge, mainly by those roads that did not obtain any Slumbercoaches...