20mm is a big deal. The length of your cranks is part of the mechanical system of levers and gears which converts your effort into forward motion. Increasing the length of the crank allows you to exert more torque around the bottom bracket, but at the cost of having to travel a greater distance. Use of two widely different cranks will result in the sense that one (or both) legs is in the wrong gear. I'm not sure if I explained that very well! Anyway. The alternative, building up the pedal and/or the cleat in an attempt to match the dimensions of the bike to an asymmetrical rider, has its own problems. Manufacturers of shoes and pedals go to enormous lengths to minimise the distance from the sole of the foot to the centre of the pedal axle. It's all about the stability of the foot on the pedal. A very tall pedal will feel 'tippy'. So, in my opinion, finding a way to compensate for your 20mm should be achieved through as many different methods as possible - in order to minimise the aforementioned negatives. Firstly, ask your other leg to sacrifice 3 or 4mm of its optimum saddle height. Then find maybe 10mm with a different crank and the rest made up through increasing the pedal axle to foot dimension (however you do it: build up the pedal, space out the cleat or thicker insoles, it all amounts up to the same thing).