NMAC BTWN A C172 AND A PIPER PA28 MAKING PRACTICE APCHS TO TMB ILS RWY 9R.

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INBOUND ON ILS RWY 9R AT TMB; LEVEL AT 2000 FT; STUDENT UNDER HOOD ON PRACTICE ILS APCH. WORKING MIA APCH WITH DISCRETE XPONDER CODE. CHEROKEE WARRIOR PASSED TO R; APPARENTLY ON ILS PRACTICE APCH; 20 TO R; 10 FT BELOW. APCH HAD GIVEN TRANSITION TO TMB TWR 118.9 -- TMB WAS USING 134.6. I HAD MADE THE SWITCH TO TMB 134.6 AND WAS MONITORING 118.9 ON COM #2. HEARD NO CALL TO OTHER ACFT BY APCH OR TMB TWR OF MY ACFT AND TMB CALLED THE OTHER ACFT TO ME AS IT PASSED TO MY R AND CROSSED IN FRONT OF ME FROM R TO L. TMB TWR 'HAS POOR/NO RADAR COVERAGE IN THAT AREA' ACCORDING TO A POST-EVENT CALL TO TMB TWR CHIEF; WHO SAID A NOTAM EXISTS TO THAT EFFECT. A CALL TO MIA AUTOMATED FSS AND A REVIEW OF MY COMMERCIAL CHART NOTAMS REVEALED NO SUCH NOTAM. THE OTHER ACFT WAS BEHIND ME (5:30 O'CLOCK POS) AND DSNDING; SO I COULD NOT HAVE SPOTTED IT UNLESS I HAD REASON TO LOOK ALMOST DIRECTLY AFT AND SLIGHT R. SUGGEST MIA APCH AND TMB TWR AGREE ON WHICH FREQ IS IN USE AT TMB -- OTHER ACFT MAY HAVE BEEN ON 118.9. IF 'POOR RADAR COVERAGE' REALLY EXISTS; ISSUE NOTAM AND MOVE RADAR COVERAGE. IF NOT; KEEP CTLRS BEING TRAINED AT TMB OFF THE MKE AT PEAK TFC PERIODS; WKENDS; BTWN XA00 AND APPROX XF00.