Lend me your brains

if you put two free wheels on a double fixed hub, both the conventional way round to work on the right hand side when you flip the wheel, your left hand side hub will now run backwards as you have flipped it over. so if you put a chain on both sides, when pedaling forwards the right hand side freewheel will engage and the left hand freewheel will clickety clickety free wheel (causing a bit of drag) if you pedal backwards or try and coast your right hand freewheel will not freewheel but the left hand one will now engage and give you drive in the reverse direction. no reason why you couldn't have different ratios on both sides but chain tension could be fun (although not that important because being freewheels you can use a chain tensioner on either side).

this would allow you to have a high forward gear and low reverse gear making for easy skidding/braking, you will also have a lot more weight and frictional losses to deal with.

IMHO it will work, but not very efficiently, and whatever you do while you are moving you have a bloody freewheel clicking away which would drive me insane!!!!

http://www.londonfgss.com/post170556-47.html

But
http://spencerwright.org/mybikes/doublechain.html

"This is not the case. When the rider exerts normal forward force on the cranks, the right side drivetrain makes the rear hub spin forward. The rear hub, in turn, spins the inside, threaded half of the left side freewheel, which causes the left side freewheel to engage, spinning the left side chain. So while the top half of the right side chain is in tension, the bottom half of the left side chain is also. The former is pulling forward, the latter is pulling backwards. In other words, the left side chainring is being pulled around by the left side freewheel. The left side freewheel is being twisted by the hub, which is being twisted by the right side freewheel, which is being twisted by the right side chainring."

...

"Furthermore, breaking-in a dual-drivetrain bike is distinctly different than breaking in a normal fixie. Specifically, as either freewheel winds slightly tighter on the hub (which happens with any threaded, ie non-freehub, system), the amount of pedal lag changes slightly. This is because as a freewheel threads onto a hub, its engagement point move slightly relative to the hub, causing them to go into or out of sync with the engagement points on the other freewheel. This makes for a much different first few rides on these bikes, as the drivetrain becomes, somewhat sporadically, more or less responsive. Again, though, this process is much safer than the break-in process on a normal fixie, where a cog can thread on far enough that it is no longer touching the lockring. In these cases the lockring is prone to completely unthreading, allowing the cog to do the same. This is not the case with the dual-drivetrain system, in which niether freewheel will ever get any looser because of any pedaling force. That's because backwards pedaling forces don't affect the threaded (inner) portion of a freewheel - only a freewheel tool can do that.
Regardless, breaking-in a dual-drivetrain bike isn't especially fun. When one freewheel threads onto the hub more (for example, as a result of a hard skid), the result is often a drivetrain which is twisting itself at both pivot points. When this happens, the top half of the right-side chain and the lower half of the left-side chain are both very tight. As a result, both the bottom bracket spindle and the rear hub are under significant torsion. This makes riding the bike very * slow; * it seems to resist any force the rider puts on it."

etc etc.

so apparently it does work, after a fashion, but it binds a bit.

edit - snap!