I'm pretty sure that the original poster is referring to a twin keel fish, and not a "twin fin" which tends to have more rocker, a curvier rail outline, thinner tail and a higher-ratio aspect fin as opposed to the Gephart fins used on keel fishes.
All responses to riding a keel are spot on....rear foot directly over or slightly in front of the keels, with emphasis on rail-to-rail driving. Also, Hank Warner fishes and quad fishes are unreal. He is a protege of Bill Caster, and has been shaping quality boards for well over 30 years.
you don't ride a traditional fish the same way that you would a typical hpsb. while most hpsb thrusters want to be turned off the back off, a traditional fish likes to be turned (& trimmed) off the front. that's my experience anyway. & the ideal place, i found, for my back foot was either slightly in front of, or directly over, the fins. you'll also want to use the rail more on your turns, rather than trying to pivot off the fin cluster. this will cause your turns to be more drawn out, but they'll also be smoother. you should also loose that stalling sensation.
I was able to ride sunday and today. your advice was right on. thanks.