Check with your dealer to see if the axles will take the strain of adding a dual wheel set up. From the photos, it looks like you have the wheels at the max setting now.
The outside tire on duals puts a lot of strain on the axle and if it is not designed to take the load, something is going to brake. If you put two tractor tires together like the little garden tractor and get them into mud like you speak of, the tread wont shed the mud and they become like racing slicks. That was a problem with big tractors back in the 60s when engine power exceeded the tires grip power, farmers wanted more traction but wanted to keep tires down to 18.4 width so they could still cultivate with them so they stuck on a set of duals. Problem was that the early tractor axles werent long enough to properly space them and the tires were basically side wall to side wall and mud would get in between them causing severe out of balance problems and was nearly impossible to get out without removing the tire. You could likely get a few inches wider with a machined spacer, but just be aware that every inch you space out the tires adds lots more leverage action to your axles shaft. IMHO ballasting the tires is more efficient and safer than trying to add dual tire. I dont think you will find it to be rutting any more and maybe even less if your tires are spinning when going thru the mud, that will dig a deeper rut than a ballasted tire that doesnt spin.