Shortly after leveling off at 14,000 ft the left engine quit. The pilot diverted to Tampa, and 34 minutes later the right engine quit. During the subsequent forced landing the airplane collided with the roof of a covered sidewalk and then trees. Teardown of the left engine revealed that the seal between the exhaust flange & turbocharger was not installed; bolt clamp was finger tight. Heat had melted wiring harness & mag grounding leads insulation; both 'P' leads were grounded. Pilot stated that after losing the left engine he initiated a left tank to right engine crossfeed, and after 20 minutes of single engine flight in this configuration the right engine quit. Flight manual states to land as soon as possible in case of one engine failure. In addition, it warns that when flying on one engine & crossfeeding fuel, absolute coordinated flight must be maintained to prevent unporting of the tank's fuel pickup tube when the tank is not full.