When mowing along long banks, one of which I have is over 500 feet long, mower-plus-rider-weight roll center compared to tire outside track width is a legitimate concern. As a mower tips further and further to either side, it approaches a threshold angle beyond which it will instantly begin a potentially fatal side roll. Many threshold effects provide user warning signs. But side rollovers don't. Once they start, it's too late to correct them. So to be prudent, mower operators should maintain a prudent safety margin angle.
Mowers without riders also have side-roll threshold angles. If you own one of them, you can test it by tipping it close to its critical no-go angle. Then display that threshold angle by some indicator which its operator can observe and guide his/her actions. That is fine and probably prudent for walk-behind mowers, some of which have cutting swaths much wider than typical 30" Comets.
But when we add the unknowables of rider size, weight and tilting driver technique variations, no rider manufacture's attorneys would approve testing those waters. Snapper's Comet Owner's Manual is very specific about NOT mowing along bank lengths, even if they are 500+ feet long. Instead we are supposed to go up and down banks over and over and . . . . . ad nausium. If you assume a 6 inch overlap between passes, that would be 250 runs up that bank. Yet I think they were wise to legally cover their tails with that suggestion. Based on some lawsuit claims, it wouldn't surprise me if some idiot figures a way to tip their running mower up on end or on its side and then stick their face in its spinning blade(s) to determine if the bottom end is showing too much play or might be seeping oil. His widow would sue the mower manufacture and some attorney would try to press the case. Why these clowns aren't disbarred is beyond me.
I can also imagine that some private person might want to regularly mow a long bank on their property. If they widen their mower's wheel base they clearly extend its tipping-threshold angle.
But even if you were to improve your Comet's side-roll threshold angle, that would not improve its engine oiling at steep angles nor address increased axle loadings. John Deere's 317 models were sold with a Kohler engine which performed very well until anyone operated one of them for a while on a side-tilting bank. The joke line I heard included double meanings from the single spoken word "dearly" or "Dearely."
"That design error cost both John Deere and Kohler 'Deerely.'"
It seems to me that if Comets were entered into tractor pulling contests, they would pull wheelies that wouldn't permit steering. So traction utility gained by adding duals doesn't seem to fit well.
How about a second "dead" axle, wider than the original rear with slightly smaller tires ,so they only touch the ground if it tips slightly..(Otherwise they might lift the driving wheels off the ground)..that wont put any strains on the original rear axle..if you made it a foot wider on each side that should really help it not to tip as easily..
"Training wheels" used during periods while kids learned how to balance a bicycle sound like the same concept. Agreed that they'd add no more load to the live axles and could widen the base. I think your idea would work.
Not sure what model you have but with my 28" the rear wheels already just barely clear the uncut line (going straight). Meaning they are in line with the outside edge of deck. If you were to add another it would make manuverability lousy around trees and any other obsticles. As you may already be aware these mowers are not know for there great TR. The part the is the worse IMO is how far back the deck is in relation to the axle. Tough to explain unless you've run one.
I don't have a problem with the TR. I think they are pretty nimble actually. If you are doing it to look cool then go for it. You could add them pretty easily i would think. would require some welding and ingenuity. A couple pieces of flat and, 3 nuts and a pipe would do it. Better yet just get some new hubs and weld them to the pipe on one side and a piece of flat on the other. If you are doing it for functionality for get it. It would be aweful on these not to mention it would flatten out the grass before you ever mow over it. That is unless you use some of the skinny tires like were used on the older models and 25" models. Then you wouldn't lose much of anything!