Dual ISO-Cog set-up?

We received an interesting enquiry regarding running a dual cog set-up similar to the very clever Surly Dingle.

Some pictures below of how this might work - showing a 16t and 18t 3/32" dual set-up. It is definitely not suitable for 1/8" cogs - too much spacing required leaving the outside cog a long way from the hub mount. However, with two 3/32" cogs separated by two 2mm ISO spacers it may be usable.

This places the cogs (on a 120mm frame spacing) at approx 41mm and 46.5mm so would match up really well with a standard road double chainset. Should work well on a mountain-bike too but would perhaps need a bit more fiddling with chainline.

However, there is still the concern that the outside cog may be putting too much twisting strain on the mounting bolts by being some 7mm from the mounting flange. 20mm bolts may be necessary - even with our longest 15mm bolts only 7mm remains in the hub flange.