Eddy Merckx had a similar problem after a crash left him with one leg longer than the other. First he tried shimming the short leg. Then he half shimmed the short leg - pushed the long side into the shoe and pulled back on the short side. Probably worked well with leather shoes. Davis Phinney was coached by Merckx and he had a naturaly short leg. He used Merckxs method of pushing the long side deep into its shoe and pushing back hard on the short. This worked best for them both.
Pro cyclists do not shim and wedge bike shoes EVER. They all use custom orthotics
British cycling and Sky cycling team were strongly opposed to this because it causes way more problems than it solves.
Ignor the idiots who pass themselves off as pro bike fitters and the clowns working in LBS. See a podiatrist for a proper assessment. Get yourself a pair of custom made orthotics. The only way to tell how much one leg is longer is by an MRI. Any other way is BS.
Normally people are able to naturally adjust to a leg difference of 6mm without any adjustment.
My advice if you are not racing is to find some of the old style cycling leather shoes. These are often sold online to the old timers - "STEEL IS REAL"
Push long side in deep and pull back hard the short side. Now v important adjust the seat forward/back position - set the long leg side knee "slightly ahead" of Knee over ball joint at 3 oclock. AND short side knee "slightly behind" knee over ball joint at 3 oclock. If you are not racing take this from in front of the knee - not middle of knee. This will give you a bit more of knee protection.
If you want to race follow a sports podiatrist advice and get a custom orthotic. If you cannot afford this go to a chemist discount store and buy a ready made :
Scholl ORTHAHEEL" ready made insole. I use these and they are the best thing I have ever bough for cycling.
Once again wave goodbye to anyone who tells you to shim or wedge a cycling shoe.