What would be the best way to run a single F20T12 on an instant start ballast for 2 F32T8s?

I think the t12 operates at a lower arc voltage than the t8 so t12 and t8 aren't electrically interchangeable. Also, the instant start ballast may overdrive the lamp and sputter the cathodes. Your best option is to get either a magnetic or rapid-start t12 ballast intended for F20T12. I cannot guarantee operating a t12 on a t8 ballast as the lamp may work for a short time but the lifespan will be reduced and the lamp may fail prematurely.

For a brief test I'd just wire it up as if the lamp was a F17T8. Most F32T8 ballasts should also run F17T8 lamps.

For long term use I'd get a proper F20 ballast though.

Double up the blue wires for "overdrive" and it'll probably run the lamp near spec. If not switched too often could last a while