PLT ALLOWED ACFT TO DESCEND BELOW ASSIGNED ALT.

Narrative

AFTER ROTATION FROM CAK AND PRIOR TO GEAR UP (F/O FLYING) IN IMC CONDITIONS; BOTH AIRSPD INDICATORS CEASED TO OPERATE. THEY BOTH INDICATED 0 AIRSPD. I CONTINUED THE IMC CLBOUT BY REF TO ATTITUDE AND PWR SETTING. THE ERRONEOUS AIRSPD INDICATIONS ACTIVATED BOTH THE OVERSPD AND STALL WARNINGS. VERY DISTRACTING AT A CRITICAL TIME! UPON REACHING VFR ON TOP CONDITIONS; THE CAPT DECIDED TO FLY THE REMAINDER OF THE LEG. HE INSTRUCTED ME TO DISARM BOTH OF THE ABOVE WARNING SYSTEMS BY PULLING THEIR CB'S. I HAD TO LEAVE MY SEAT TO DO SO. SHORTLY AFTER LEVELING AT OUR ASSIGNED ALT OF FL290; THE CAPT INSTRUCTED ME TO CONTACT COMPANY RADIO; INITIATE A PHONE PATCH WITH ANOTHER GND FAC; OBTAIN WX FOR ARPTS AS POSSIBLE ALTERNATES (VFR CONDITIONS); CONSULT THE ACFT SYSTEMS MANUAL AND RUN THE APPROPRIATE EMER CHKLISTS. WHILE INVOLVED WITH THE ABOVE; THE #2 GEN WENT OFF LINE. AT THIS POINT THE AUTOPLT WAS DEACTIVATED AND THE CAPT WAS HAND FLYING THE ACFT. I LOOKED UP FROM THE ABOVE ASSIGNED DUTIES TO SCAN THE INSTRUMENTS. WE WERE SLOWLY DEVIATING FROM ALT. I CALLED 'ALT' TO THE CAPT; WHICH HE ACKNOWLEDGED. HOWEVER; THE CAPT MISINTERPRETED THE ALT READOUT AND DSND RATHER THAN CLBED TO CORRECT. ATC INQUIRED WHAT ALT WE INDICATED AND THE CAPT REPLIED; THEN CORRECTED FROM FL280 TO FL290.