PLT OBSERVER CHALLENGES PIC ABOUT RWY AWARENESS AS HE INCURS RWY DURING TRAINING SESSION.

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AN ORIENTATION FLT WAS CONDUCTED FROM FXE TO MLB AND BACK TO FXE. AN IFR FLT PLAN HAD BEEN FILED BY THE PIC FROM MLB TO FXE. THE PLT HAD FLOWN THIS ACFT SEVERAL TIMES IN THE PAST AND HAD BEEN FLYING MORE SOPHISTICATED EQUIP; SPECIFICALLY A BEECH 1900 UNDER FAR PART 121 RULES. I WAS FLYING TO GET HIM REACQUAINTED WITH THE PIPER AZTEC PA23-250. DURING ENG START; WE FELT PRESSURED TO DEPART ASAP. IN ORDER TO SHARE THE WORKLOAD; I AGREED TO HANDLE THE RADIO COMS DURING TAXI. SINCE WE WERE UNFAMILIAR WITH THE ARPT; WE REQUESTED PROGRESSIVE TAXI INSTRUCTIONS TO THE ACTIVE RWY FOR OUR IFR DEP. WE RECEIVED THE TAXI INSTRUCTIONS; WHICH THE PIC ALSO HEARD AND ACKNOWLEDGED THROUGH THE INTERPHONE SYS. AS WE GOT CLOSE TO THE RWY (ABOUT 25 FT); I INFORMED HIM THAT WE WERE APCHING THE RWY 4/TXWY C INTXN AND SUGGESTED THAT WE START SLOWING DOWN TO STOP. HE ACKNOWLEDGED BY INDICATING THAT WE WERE CLRED TO TAXI TO RWY 4. AT THIS POINT; I INDICATED THAT WE WERE APCHING THE ACTUAL RWY 4. I PLEADED WITH HIM TO SLOW DOWN AND STOP THE AIRPLANE SINCE IT APPEARED WE WERE ABOUT TO CROSS THE RWY. HIS RESPONSE WAS THAT 'WE WERE CLRED TO TAXI TO RWY 4.' AFTER THIS RESPONSE; I SHOUTED THAT WE NEEDED TO STOP THE ACFT IMMEDIATELY. INSTINCTIVELY; I TRIED PRESSING ON THE BRAKE PEDALS. I RETARDED THE THROTTLES IN A VAIN ATTEMPT AT STOPPING THE ACFT WHILE TRYING TO TURN AWAY. HOWEVER; THE R SIDE PLT POS HAS ONLY RUDDER PEDALS AND NO TOE-BRAKES. CONSEQUENTLY; DESPITE MY PLEADING WITH THE PIC; RETARDING THE THROTTLES; AND PRESSING PERSISTENTLY ON NON EXISTENT BRAKES FROM THE R SEAT; THE ACFT TAXIED ONTO THE ACTIVE RWY 4. AT THIS POINT THE PIC IN THE L SEAT TRIED TO STOP THE ACFT IN THE MIDDLE OF THE RWY WHEN HE REALIZED HE WAS ON A RWY AND NOT A TXWY. RATHER THAN SIT THERE AND TAKE THE CHANCE AN ACFT MIGHT LAND ON TOP OF US; AND NOTING THAT THE TXWY IN FRONT OF US WAS CLR; I PROCEEDED TO ADD PWR TO CONTINUE TAXIING TO GET OUT OF HARMS WAY IF INDEED IT WAS THERE. WE MANAGED TO REACH THE OTHER TXWY AND MADE A 180 DEG TURN. ONCE OUR RUN-UP WAS COMPLETED AND WE DEPARTED MLB; IT WAS NOTED BY THE PIC AND I THAT THERE WAS SOME SERIOUS CONFUSION. HE STATED SEVERAL TIMES THAT I WAS HANDLING THE RADIOS AND DIDN'T UNDERSTAND WHY THIS HAD HAPPENED. I AGREED AND INFORMED HIM OF THE CLRNC WHICH HE HAD PREVIOUSLY STATED HE UNDERSTOOD; YET HE HAD NOT ADHERED TO MY REQUEST TO STOP THE ACFT WHEN WE WERE APCHING THE RWY. CONTRIBUTING FACTORS TO THE EVENT WERE UNFAMILIARITY WITH THE FIELD AND MISUNDERSTANDING OF THE TAXI INSTRUCTIONS BY THE PIC. FURTHERMORE; ABSOLUTE AWARENESS MUST BE MAINTAINED WHEN APCHING ANY INTXN. DESPITE MY BEST EFFORTS AT PREVENTING THE INCIDENT; I AM NOW EMBARRASSED BUT EVER SO WISER AND MORE VIGILANT. I WILL NOT TRUST ANYONE IN THE L SEAT REGARDLESS OF THEIR PAST EXPERIENCE AND/OR FAMILIARITY WITH THE ACFT. HAD THE PIC LISTENED TO ME AND STOPPED THE ACFT; OR AT LEAST SLOWED DOWN -- OR AT THE VERY LEAST QUERIED ME -- THIS WHOLE INCIDENT COULD HAVE BEEN AVOIDED. I SHOULD HAVE BEEN EVEN MORE ASSERTIVE TO GET THE POINT ACROSS THAT WE WERE APCHING THE ACTIVE RWY. AFTER CAREFUL REFLECTION; I NOW REALIZE THE IMPORTANCE OF DUAL CTL BRAKES IN A TRAINING ENVIRONMENT. ALTHOUGH THIS WAS NOT TECHNICALLY A TRAINING FLT; THIS EVENT COULD HAVE BEEN COMPLETELY AVOIDED IF THE ACFT HAD BEEN FITTED WITH DUAL BRAKES SO THAT I COULD HAVE REVERTED FROM AN OBSERVER TO A CONTRIBUTING PLT. MY ACTIONS WERE FUTILE; SINCE I COULD NOT TAKE COMMAND OF THE ACFT AT THE MOST CRITICAL POINT.