LJ35 MIL CREW HAD AN ALTDEV IN ZID CLASS A.

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ON DSCNT INTO SCOTT AFB (ABOUT 150 NM E) ZID CLRED US TO DSND FROM FL350 TO FL310. UPON PASSING APPROX FL320 WE WERE ISSUED A TA FOR TFC '2-3 O'CLOCK POS; FL290; HEADING S.' I (BEING THE INSTRUCTOR ACFT COMMANDER IN THE R SEAT) BEGAN CLRING FOR TFC; WHILE THE UPGRADE ACFT COMMANDER IN L SEAT CONTINUED HIS AUTOPLT COUPLED DSCNT. WHEN I DID NOT IMMEDIATELY SPOT THE TFC; THE PF TRANSITIONED HIS ATTN OUTSIDE. DURING THIS PERIOD THE PF MOST LIKELY DISENGAGED THE AUTOPLT ALT CAPTURE MODE BY ACCIDENTALLY TAPPING THE 'CTL WHEEL TRIM SWITCH' WHICH IS LOCATED ON THE YOKE. WHEN I LOOKED BACK INSIDE I IMMEDIATELY NOTICED WE HAD CONTINUED OUR DSCNT TO FL303. I COMMANDED THE PF TO CLB IMMEDIATELY; AND WE DID SO. ATC HAD NOTICED THE DEV AND WAS JUSTIFIABLY CONCERNED DUE TO XING TFC AT FL290. WHEN THEY REQUESTED OUR ALT I INFORMED THEM THAT WE WERE 'CLBING TO FL310' AND THEN; 'PASSING FL306 FOR FL310.' DURING THE TIME WE DEVIATED WE HAD ALREADY CALLED VISUAL WITH THE TFC; AND THEY CALLED VISUAL WITH US. THERE WAS NO DANGER OR POTENTIAL CONFLICT. HUMAN PERFORMANCE CONSIDERATIONS: ONE PLT (PF) SHOULD ALWAYS BE 'FLYING THE JET.' THIS NEEDS TO BE EMPHASIZED THROUGH CRM. BOTH PLTS WOKE UP AT XA00 TO BEGIN DUTY PERIOD...FATIGUE.