ALT DEV ALT OVERSHOT.

Narrative

DEPARTED TRUCKEE; CA; MAINT FACILITY AFTER REPLACING OLD (LEAKING) TIP TANK. NEW TANK INITIALLY LEAKED BUT AFTER PLACING NEW SEALS AND GND TESTING IT SHOWED NO EVIDENCE OF LEAKS. TKOF WAS UNEVENTFUL AND WE CLBED TO THE SQUAW VALLEY VOR; XING AT APPROX 12000 FT. THERE WE CONTACTED OAKLAND CENTER TO OPEN OUR IFR FLT PLAN TO VAN NUYS; CA. THE PLANE WAS BEING FLOWN BY A COPLT IN THE REAR SEAT; USING HIS OWN ALTIMETER (THE ACFT IS A TANDEM SEAT EXMIL TURBO JET TRAINER). THE COPLT WAS UP FOR AN ORIENTATION FLT AND WAS UNFAMILIAR WITH THE ACFT. XING SQUAW VALLEY VOR I PULLED OUT A MAP TO CHK THE COPLT CALCULATION FOR HDG (WE WERE RNAV TO FRIANT VOR; NOT ON AIRWAY V OR J). CENTER CLRED US TO 19000 (FINAL TO BE 2500). AFTER PASSING THROUGH 1600 FT THE COPLT INFORMED ME OF FUEL STREAMING FROM THE SAME (STARBOARD) TIP TANK. WE DID SOME BANKING TO DETERMINE THE LOSS RATE (SETTING SUN WAS AT THAT WING TIP MAKING VISUALIZATION DIFFICULT). WE DISCUSSED DECLARING AN EMER. I STARTED CHANGING PRESSURIZATION FROM BOTH TIPS TO STARBOARD ONLY; PORT ONLY; AND THEN OFF TO SEE THE EFFECT ON FUEL LOSS. AT THAT POINT; ATC SAID WE WERE OVER OUR ASSIGNED 19000 FT LEVEL AND TO DSND BACK TO IT. I TOOK CTL AND WE DSNDED. I ALSO RESET THE ALTIMETER TO 29.92 WHICH I HAD NEGLECTED TO DO DUE TO ALL THE OTHER EXCITEMENT. (THE BACK SEAT ALTIMETER IS IN MILLIBAR AND HAD NOT BEEN RESET FOR FLT ABOVE 18000). WHEN MY ALTIMETER WAS RESET IT DID NOT SHOW MORE THAN 200 FT ABOVE ASSIGNED ALT. THEN ANOTHER VOICE ON ATC GAVE US A PHONE NUMBER TO BE CALLED FOR A 'POSSIBLE PLT VIOLATION.' WE CONTINUED FLT WITHOUT DECLARING EMER BY USING OUR STARBOARD TANK FUEL FIRST TO MINIMIZE FUEL TOTAL LOSS. THIS RESULTED IN A SOMEWHAT L WING HVY CONDITION MIDJOURNEY; WHICH WAS TIRING TO FLY; BUT WHICH HAD EQUALIZED BY THE END OF THE TRIP WITH A TOTAL LOSS ESTIMATED AT LESS THAN 20 GALLONS; ALLOWING PLENTY FOR IFR RESERVES. THE PROBLEMS AS I SAW IT WERE #1 DISTR BY BOTH PLTS FROM 'ROUTINE' DUTIES BY A POTENTIALLY CATASTROPHIC SITUATION AND #2 SEPARATE ALTIMETERS IN A TANDEM SEAT ACFT WITHOUT TIMELY RESETTING OF SUCH DUE TO #1.