B737 RECEIVED TCASII RA; EXECUTES GAR DURING VISUAL SIDE-BY OPS AT HOU.

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I WAS MONITORING FROM THE FO SEAT AND THE CAPT WAS FLYING. WE WERE VECTORED IN FOR A VISUAL APCH TO RWY 12R AT HOBBY. WE SLOWED DOWN; IN ACCORDANCE WITH TWR'S DIRECTIVE; FROM 250 KTS TO 210 KTS TO 190 KTS THEN TO 150 KTS. AT ABOUT 2000 FT MSL WE RECEIVED A TCASII TA STATING 'TFC.' THE CAPT VISUALLY ACQUIRED THE TWIN ENG ACFT DOING A VISUAL TO RWY 12L. THE TCASII SHOWED THE ACFT WAS 500 FT BELOW US AND THE CAPT SAID THE ACFT WAS ABEAM HIM IN A RTE POS. WE CONTINUED WITH THE APCH WITH THE CAPT KEEPING VISUAL ON THE OTHER ACFT (I COULD NOT SEE IT). THEN WE RECEIVED A TCASII RA STATING 'REDUCE DSCNT; REDUCE DSCNT.' THE CAPT FOLLOWED THE RA AND TEMPORARILY LEVELED OFF. THIS CAUSED US TO START GETTING A LITTLE HIGH ON GS SO WE CONFIGURED TO FLAPS 25 DEGS +30. AT AROUND 1300 FT MSL WE RECEIVED A TCASII RA STATING 'CLB; CLB.' THE CAPT APPLIED THE APPROPRIATE PROCS AND WE EXECUTED A GAR. THE CAPT HAD VISUAL ON THE ACFT THE WHOLE TIME; BUT DUE TO THE TCASII RA; WE EXECUTED THE GAR AND I NOTIFIED TWR OF OUR GAR DUE TO THE TCASII RA. WE WERE VECTORED AROUND FOR ANOTHER VISUAL TO RWY 12R AND LANDED. AFTER LNDG; WE WERE INFORMED TO NOTIFY THE TWR. THE CAPT TOLD THE TWR THAT WE USED TO BE ABLE TO MAINTAIN VISUAL WITH A TCASII RA; BUT NOW DUE TO COMPANY MANUAL ARE NOT AND MUST IMMEDIATELY RESPOND TO TCASII RA'S. AT NO TIME WERE WE IN ACTUAL DANGER OF HITTING THE OTHER ACFT.