One of the reasons I do not like duo's and that they are totally unnecessary in their concept. Is a myth that does not seem to want to die.
You can most definitely fill in the gaps. All you need is to buy separate cross chains from Tire Chains. Just measure the spread on yours while mounted. Come with hooks and all. On their website, they have descriptions of sizes. In the end, you'll have chains that are more uniform and grip better as a result.
My actual experience was different. I originally tried 4-link ladders and they were ineffective for my 43x16-20 R4's. Most of the cross chains got lost in the tread. The DUO's solved that but were too rough. Part of the problem is that my tires are actually a skid steer tire and not all that common. The fairly small running diameter and patterns which were 1-chain link too long resulted in large gaps between the patterns. The supplier
www.canadianchains.com actually changed the cross link pattern of my DUOs to a shorter one the year after I bought mine (but I bought mine from a different seller). I fixed that roughness by ordering extra DUO patterns and converting the DUOs to H-pattern and as shown in the thread linked in my post #3 of this thread. But even without that change the DUOs were much more effective than the ladders.
I had a large end-gap (4-side chain link gap) on my now H-pattern (formerly DUO) chains which I filled with a straight cross chain as you suggest. After a while the chains would shift badly at the end gap. I solved that by adding another DUO pattern across the end gap. The photos are in the thread referenced in my post 3 above but I'll post them here:
Chains with straight cross chain at end gap:
Replace straight cross chain with DUO pattern (left tire), right tire DUO pattern not yet added:
That stopped the shifting problem and improved traction/ride over the end gap.
I suspect I would not have had the shifting problem if I could have followed your recommendations and ran the chains loose. However I have less than 2" between the side chain and ROPS support bracket so I needed to keep the rear chains tight. The tensioners likely were the cause of the chains shifting. I had a similar clearance problem with my front chains and could not even mount chains until I got 2" spacers. The front chains are small H-patterns which I run fairly loose (just one bungee in a triangle pattern connecting 3 points on the side chain) because there is now good clearance.
I am finally going to fix the rear chain clearance by adding spacers. I got a quote from Bro-tek before Christmas and intend to order them soon. I'll put the spacers on in the spring because I don't want to mess with them with all the snow. I'm not confident I can handle the loaded rear tires so I will get a tire service company to install the spacers after I take the chains off. So next year winter I won't have to fuss with the rear tire tight clearance and will run them looser.
Had I been able to run my original 4-link ladders looser I may not have had the problem with them sinking in the tread and they would have been fine. If I had installed spacers from the start I may have ended up with 2-link ladders on the rears instead of trying DUOs. The DUOs were very rough but converting them to H-patterns fixed that and I suspect they are somewhat more effective than ladders.
I was fortunate to have a local supplier who stocks bulk chain patterns at about 1/2 the price of the online stores so I could modify my chains at a reasonable cost.
