FLC OF LGT INSTRUCTED TO KEEP FAA NAVAID INSPECTION ACFT IN SIGHT. LOST SIGHT OF ACFT AND SAW HIM PASS UNDER NOSE SEVERAL HUNDRED FT BELOW.

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APCHING ANDREWS AIR FORCE BASE IN VISUAL CONDITIONS AN SMT ON A FLT CHK OF THE ILS TO RWY 19R WAS RPTED TO US FOR VISUAL CONTACT. AFTER LOCATING THE SMT WE RPTED IT AND THE FIELD IN SIGHT FOR A VISUAL APCH. THE CLRNC FOR VISUAL APCH TO RWY 19R WAS ISSUED WITH THE REQUIREMENT TO KEEP THE SMT WHO WAS BELOW US; IN SIGHT. THE SMT WAS ON A DIFFERENT RADIO FREQ AND WE WERE CONCERNED REGARDS HIS INTENTIONS AS WE WERE NEARLY MATCHED IN SPD AND LOCATION ON APCH TO THE SAME RWY. AFTER SOME DISCUSSION WITH THE TWR WE DETERMINED THAT THE SMT WAS TO TURN OFF THE LOC 1.5 MI INSIDE THE MARKER. DURING THIS CONVERSATION WE LOST SIGHT OF THE SMT UNDER OUR NOSE AND SHORTLY AFTERWARDS SAW HIM IN A L TURN ONLY A FEW HUNDRED FT BELOW US. IT WAS OUR RESPONSIBILITY TO KEEP THE SMT IN SIGHT BUT LOST HIM DUE TO CLOSE PROX AND OTHER COCKPIT WORKLOAD. HOWEVER; THE SITUATION WAS PRECIPITATED BY THE APPARENT INSISTENCE OF THE SMT TO CONTINUE THE FLT CHK WITH OTHER ARRIVING ACFT ON THE APCH AND THE CLRNC FROM THE TWR TO 2 DIFFERENT ACFT TO BE IN THE SAME AIRSPACE AT THE SAME TIME. IN RETROSPECT; I SHOULD HAVE MORE SIGNIFICANTLY MODIFIED MY APCH TO MAINTAIN A LARGER VISUAL SEPARATION.