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There are 5 and 6 speed transmissions that have very wide gear spread already. The modified NV4500 I have in my Suburban has the following ratios: 5.61-3.04-1.67-1.00-0.643 which is a 8.72 gear spread.
With 4.10's first is unnecessary for a torque down low engine, so I take off in second. Thus the vehicle could support a much taller rear end ratio and still take off fine in first, to the point that the overdrive ratio could be pushed to the limit of the engine's torque curve versus road load. For example, with 235/85R16 tires and relatively common 3.42 gears I would be turning 1208 rpm at 55 MPH. All with only one transmission and no auxiliary devices. With 3.08 gears this would be 1088 RPM. This is probably beyond the reasonably limit of most engines without drag and weight reduction to reduce the road load.
For 2wd applications there is the Tremec 6060 with 2.97 first and 0.50 overdrive. Less gear spread but more intermediate gears.
So given there are existing single transmissions with such spreads, I don't see the point in using multiple boxes outside of specialized use cases (e.g. rock crawling).
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