A319 FLT SLOWED BELOW VLS DURING VISUAL APCH TO DFW.

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APCHING DFW WITH WX N AND W. BRIEFED RWY 18R AT TOP OF DSCNT. AFTER CHKING WITH APCH AND LOOKING AT RADAR; REQUESTED RWY 17L TO PROVIDE MOST DISTANCE FROM WX. APCH CONCURRED. AT APPROX 10000 FT OVER DFW; APCH CLRED US TO 6000 FT AND REQUESTED EXPEDITE TO 'BEAT WX TO THE FIELD.' CAPT SUGGESTED GEAR DOWN; WHICH WE DID AND EXPEDITED AT 230-240 KTS. APPROX ABEAM END OF RWY AND PRIOR TO LEVELING; WE WERE CLRED TO 4000 FT AND SHORTLY AFTER TO 3000 FT. CAPT INDICATED TO APCH WE HAD FIELD IN SIGHT. APCH CLRED US FOR VISUAL AND DOWN TO 2000 FT. CAPT INITIATED HDG SELECTED TO APPROX 230 DEGS. AT THIS POINT; WE WERE ON A FAIRLY CLOSE IN DOWNWIND AND I KNEW WE WOULD HAVE A BUSY APCH. DURING TURN TO FINAL; THERE WAS SOME MISCOM WITH CAPT. I PLANNED ON INTERCEPTING FINAL OUTSIDE FAF AND ON LOC. I WAS DIVIDING MY TIME LOOKING OUTSIDE AND IN; AS I HAD THE ARPT IN SIGHT. WE WERE STILL ON AUTOPLT AND IN L TURN WHEN CAPT SAID TO KEEP IT TURNING AND SUGGESTED I 'CLICK IT OFF' (APPARENTLY MEANING AUTOPLT). I DISCONNECTED AUTOPLT JUST PRIOR TO AND AS WE WERE LEVELING AND CONTINUING INTERCEPT AT APPROX 25-30 DEG BANK. WE ALSO NEEDED TO SLOW RAPIDLY FOR FLAP EXTENSION; SO I PULLED THROTTLES BACK (DISCONNECTING AUTOPLT). AT THIS POINT; I HEARD 'DUAL INPUT' AND CAPT INDICATING TO KEEP IT TURNING. HE WAS CONCERNED ABOUT WX TO W OF FINAL. UNFORTUNATELY; BTWN LOOKING OUTSIDE; QUICKLY LEVELING; 'DUAL INPUT' CALL; CAPT'S COMMENT; AND NEED TO SLOW; I FAILED TO RETRACT SPD BRAKE AND MAINTAIN FULL SITUATIONAL AWARENESS OF SPD. SPD SLOWED BELOW VLS. I NOTICED AT SAME TIME CAPT CALLED 'SPD' I HAD ADDED SOME PWR; BUT NOT ENOUGH. AS I AGGRESSIVELY ADDED PWR; CAPT RETRACTED SPD BRAKE AND SELECTED FLAPS 1 DEG. WE THEN ACCELERATED TO MANAGED SPD; INTERCEPTED GS AND PROCEEDED REMAINDER OF APCH STABLE. DURING FLARE WE ENCOUNTERED SOME ROUGH AIR AND SOME FLOAT. AS I LANDED IN TOUCHDOWN ZONE; I AGAIN HEARD 'DUAL INPUT.' CAPT HAD APPARENTLY ADDED NOSE DOWN TO ENSURE WE GOT AND KEPT NOSE DOWN. I INITIALLY THOUGHT CAPT 'DUAL INPUT' MAY HAVE BEEN ACCIDENTAL DUE TO CAPT IN READY POS AND TURB. IT WAS ALSO CLR THAT HE WANTED MORE BANK DURING TURN. WE EXTENSIVELY DEBRIEFED AFTER FLT. LOOKING BACK; THE SUGGESTION BY ATC THAT WE EXPEDITE FOR WX INITIALLY MADE ME THINK; SHOULD WE BE RUSHING TO BEAT WX AND DID NOT ADVOCATE STRONGLY TO QUESTION THIS. WHEN WE WERE ON DOWNWIND; I THOUGHT AND MENTIONED WE MIGHT WIDEN/EXTEND; BUT AS I MENTIONED; WE WERE CLRED FOR VISUAL AND AGAIN; I COULD HAVE ADVOCATED STRONGLY. DURING TURN TO FINAL; I LET THE RAPID PACE AND DISTRS OF WX; ATC AND UNUSUAL DUAL INPUT DISTRACT ME FROM THE TASK AT HAND. THIS RESULTED IN SLOW AIRSPD. I ALSO SHOULD HAVE RECOGNIZED THAT THE CAPT WAS IN A BETTER POS FOR A VISUAL ON L SIDE OF ACFT AND BETTER VIEW OF WX AND RWY. I SHOULD HAVE SUGGESTED HE FLY THE VISUAL. LASTLY; THIS WAS THE 4TH LEG OF A VERY LONG DUTY DAY AND I SHOULD HAVE RECOGNIZED MY LIMITATIONS AND INDICATED DESIRE FOR MORE LEISURELY APCH. I LEARNED A GREAT DEAL FROM THIS AND WILL CARRY MANY TOOLS WITH ME TO FUTURE FLTS. SUPPLEMENTAL INFO FROM ACN 619517: THE DUAL INPUTS RESULTED IN MY ATTEMPT TO ASSIST WITHOUT RELINQUISHING CTL OF THE ACFT; ALLOWING THE FO LATITUDE TO FLY THE ARR WHEN IN RETROSPECT I SHOULD HAVE TAKEN CTL SINCE THE ARPT WAS ON MY SIDE.