White Industries went one (two?) better way back during the 00s single speed craze. A two speed freewheel and a “double” single speed chaining, so you could which between a 38/16 and a 36/18 without changing the chain length or having to move the axle too far - high gear to get you to the trailhead, then low gear to get through the technical or uphill stuff. They called it Dinglespeed!
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Retro direct! Super rad and yes, super confusing.
ETA: I only know this because back when I was working at a shop a buddy who worked there made one with a bunch of spare parts!
Found on French bikes around 1900. Very confusing when you see someone pedalling backwards when going uphill.
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A local friend built up a retro-direct using an old elevated chainstay MTB and I got to play on it a few times. Pretty fun and sure seems odd at first. Forward for the high gear and backwards for the low gear. Sure turns heads when people see you going up a hill pedaling backwards 
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I would it set up pedalling backwards in the high gear, since the low gear will be used for starting off, climbing, technical stuff etc, when you need better control and higher peak torque.
For those unfamiliar with the concept, the chain goes from the bottom of the chainring upwards to the top of the freewheel, then around a pulley wheel mounted on the chainstay, then back to the smaller cog on the freewheel and along the top of the chainring. The key is that the two freewheel sprockets are independent, so when one is being driven by the chainring, the other is coasting. It’s very hard to put into words!
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I hadn’t heard of this french reverse reduction gear things Retro-direct - Wikipedia
Doesn’t sound like a bad idea as you don’t incur as much friction as other reduction gears, which would require two gear interfaces instead of one here. Plus, it allows you to use a second set of muscles if needed.