From discussions I've heard about it I don't think that's it at all. For some reason duals tend to dig themselves in faster in sand. I'm no expert and all I have to share is hearsay from folks who are 'experts', such as on Matt's Off Road Recovery who operate in those conditions all the time. But what I've heard from those folks contradicts this idea.
Carrying ability, especially off road on low air pressure, has little to with tire width, but with air volume.
We used to have a tractor on 12.4-36 tires, with add-on duals.
Our later one had 16.9-30 which are the same height, yet 50% wider. They had the same load capacity as the 12.4-36 on duals.
Also modern tractors, when you go over 200hp tractors dont use the wide ratio 650/65R42 anymore, but 650/85R38, higher sidewall gives more air volume, gives more flex, thereby a larger contact patch and more carrying capacity.
Single tires have high sidewalls, and therefor are better in bellying out over a large contact patch, by making contact to the ground over a longer length, without increasing the amound of mud the tire has to displace to get along.
Here in Holland we run dumptrucks on supersingles all around, even the fixed axle. 11.5 ton allowed when the axle has permanent drive and is more than 6 feet spaced from the next axle, with a max GVW of 50 ton.
When you create ruts, you have less resistance with all wheels in the same track. With a conventional 8x8 you run a track with your front axles, then your tandem duals push a new rut in the soil on both sides of the front axle rut. Displacing soil causes resistance.
Heres a Tatra 10x10 with a legal gross of 50 ton, in dredger sand. Since these became available, more and more people choose supersingles on dumptrucks.