THE OPERATOR HAD APPLIED FOR AN ON-DEMAND/AIR TAXI CERT, BUT HAD NOT OBTAINED APPROVAL. AFTER LVL-OFF ON A FLT TO CARRY CANCELED CHECKS, THE PLT ELECTED TO USE THE REMAINING FUEL FM THE INBD TANKS. HE STATED THAT SOON THEREAFTER, THE LEFT ENG BGN RUNNING ROUGH, SO HE SELECTED THE OTBD MAIN 'ON THE LEFT SIDE WITH THE BOOST PUMP ON AND NOTHING HAPPENED.' HESAID HE THEN PSND THE L FUEL SELECTOR TO THE INBD TANK & NORMAL ENG OPN RETURNED, MOMENTARILY. AN IDENTICAL SCENARIO OCCURRED ON THE RGT SIDE. AFTER A DUAL PWR LOSS, THE PLT TRIED TO RTRN BACK TO THE DEP ARPT, BUT WAS UNABLE. HE LNDD IN A FIELD, BUT THE ACFT HIT TREES BEFORE STOPPING. AN EXAM OF THE ACFT REVEALED THE INBD TANKS WERE EMPTY, BUT THE OTBD TANKS WERE FULL OF FUEL. THE PSN OF THE FUEL SELECTORS (WHEN THE ENGS LOST POWER) WAS NOT DETERMINED. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FND. THE FUEL SELECTOR PANEL WAS ON THE FLOOR BTN THE PLT & COPLT SEATS. DUE TO RE-UPHOLSTER-ING OF THE SEATS, THE PLT COULD NOT SEE THE PANEL UNLESS HE PULLED THE SEAT CUSHIONS APART.