Perhaps Cooper used the dummy reserve to pack a portion of the ransom. After all, the money bag was completely filled and would be easier to tie off at the top if some of the money was transferred to the dummy reserve.
I always assumed this as well because it would've have been more durable than the bank sack. But why not use both both reserve chute containers? Seems like an unnecessary risk to use the sack at all.
Why guess about this? Why do people prefer guessing over testimony? Guessing is more exciting and you can make shit up! Maybe people will be discussing 'made up shit' 200 years from now then the poser can say: "I influenced civilisation" on his grave!

But here's the facts:
Cooper Knapsack and Money Packing –
2310 – [Pilot Notes] wants money in negotiable currency to be passed & a crew member . [Nothing in Pilot Notes about knapsack]
3:13pm 305: Passenger advises is hijacking enroute to Seattle. Stewardess (Schaffner) has been handed note requesting 200,000 and knapsack by 5:00pm this afternoon. Wants 2 back pack parachutes. Wants money in negotiable American currency denomination of bills not important. Has bomb in briefcase and will use if anything is done to block his request.
3:20pm
He must have knapsack with money before any other steps taken...(7:41pm pst) NWA - Stewardess Mucklow allowed to go to cockpit.
7:42 pm t1
305: MSP FLT OPS this is 305 outbound Seattle 14 miles (out) on V23.
Seattle he is already trying to get the door down. Stewardess (Mucklow) is with us (in cockpit). He cannot get the stairs down.
305: We now have an aft stair lite on.
MSP: Roger.305.
7:54 pm t1
MSP: As soon as reasonably sure the man has left the quicker you can land.
305: Roger.
Miss Mucklow said he apparently has the knapsack around him and thinks he will attempt a jump. Florence Schaffner 11/24/71
Cooper remarked about how heavy the money was. He seemed amused and child-like.
Schaffner furnished 13 pages of notes which she took during the course of the flight.
[No statement about knapsack]
Interview 11-30 Mucklow:
Mucklow then used a plain envelope to write out the demands of the hijacker, listing that he wanted four parachutes including two back packs and two chest packs, $200,000 in cash in small bills, and that he wanted everything by “by five o’clock”. ” Mucklow says that Florence Schaffner delivered this note to the pilot’s compartment. [Nothing about knapsack]
The hijacker displayed an extensive knowledge of the aircraft and seemed specifically well informed in refueling procedures to the point that the crew had difficulty in convincing him that only 90% of the required fuel was on board at the time he was protesting the fact that refueling had not been completed.
It was also during this time that he complained to Mucklow that he had requested the money be delivered in a knapsack but instead it was delivered in a cloth type bank bag, which displeased him. It was at this time that Mucklow recalls he stated he would be forced to use one of the parachutes to rewrap the money since he had not been furnished the knapsack. At this same time Mucklow says she suddenly observed him having a small green paper bag, contents unknown.
Mucklow received the impression that this hijacking was carefully planned and thought out in advance in that the hijacker was even specifying that the money was to be furnished in a knapsack and even had already insisted that a discarded matchbook cover (he had provided for one of the stews to light a cigarette for him) be returned to him (and…
Interview of Mucklow 12/1-2 at her home in PA:
The hijacker later told (repeated to) Mucklow that he wanted $200,000 in circulated US currency, two back and two front parachutes, and fuel trucks to meet the plane when it landed.
When Mucklow returned to the plane with the last back pack chute, she saw that the hijacker had one of the small chutes open and was cutting nylon cords out with his pocket knife. He took the nylon cord and wrapped it around the neck of the money bag numerous times and then he wrapped it a few times from top to bottom, and with the same piece (of cord) he made a loop like a handle at the top. This nylon cord was pinkish in color. He appeared irritated that they hadn’t given him a knapsack for the money as requested, and after trying to put the money in an unfolded parachute, he decided to leave it in the canvas bag (and fabricate a holding line for that, instead). He said, “We’re going to Mexico City, gear down, flaps down, you can trim the plane to 15, you can stop anywhere in Mexico to refuel, but not here in the United States. The aft door must be open and the stairs down. The altitude, under 10,000 feet, they know they can’t go over that. Cabin lights out and everyone is to be forward of the first class curtain.”
?> original request two back pack chutes .... may be added two front pack reserves as insurance in case the knapsack for hauling money got overlooked?