TWO HELICOPTERS WERE BEING USED IN LAW ENFORCEMENT ACTIVITIES. ONE WAS A LOS ANGELES SHERIFF'S DEPT (LASD) MCDONNELL- DOUGLAS 369E, N451DS, CALL SIGN AIR 21. THE OTR WAS A COMPTON POLICE DEPT (CPD) ENSTROM F-28F, N48HR, CALL SIGN AIR 28. WHILE IN THE LAX TCA, AIR 21 RESPONDED TO AN EMERG CALL. AFTER DEPG THE TCA, THE PLT (AIR 21) SWITCHED HIS COM RADIO TO 123.02 MHZ (COMMON FREQ FOR LAW ENFORCEMENT IN THE AREA) & ANNOUNCED HIS PSN. WHEN NO RESPONSE WAS RCVD, HE SWITCHED TO THE COMPTON UNICOM FREQ (122.9 MHZ). AT ABOUT THAT TIME, AIR 28 WAS ORBITING IN THE SAME VCNTY. THE PLT OF AIR 28 SAID HE HAD ANNOUNCED HIS PSN ON 122.9 MHZ (RQRD FREQ IN THE AREA FOR CPD ACFT) & WAS MONITORING 123.02 MHZ. THE 2 CONVERGED & COLLIDED, BUT BOTH REMAINED CONTROLLABLE & WERE LNDD FROM EMERG APCHS. BFR THE MIDAIR, THE PLT OF AIR 21 WAS HELPING HIS OBSERVER LOOK FOR A PSN ON A MAP. HE SAW AIR 28 JUST BFR IMPACT, BUT NOT IN TIME TO AVOID A COLLISION. THE PLT OF AIR 28 HAD HIS TRANSPONDER ON STANDBY, ALTHO HE WAS REQD TO BE OPERG IT, WHILE WITHIN 30 MI OF THE (GROUP 1) TCA.