PARALLEL VISUAL APCH SIT IN WHICH LEAD ACFT (ACR X IS TO MAINTAIN VISUAL SEPARATION WITH ACR Z).

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ON THE QUIET BRIDGE VISUAL TO SFO RWY 28R APCH CTL CALLED OUT AN ACFT AHEAD. BELOW AND TO OUR L ALSO ON APCH. WE DID NOT SIGHT THE ACFT AND RESPONDED TO THE CTLR NEGATIVE CONTACT. CTLR THEN CLRED OUR ACFT TO MAINTAIN 2500 FT AND 250 KTS. OUR ACFT WAS NOW APPROX 7 DME FROM RWY 28R. WE QUERIED THE CTLR ABOUT THE AIRSPD HE WANTED US TO MAINTAIN AND HE REPLIED HIS INTENTIONS WERE TO HAVE US OVERTAKE THE ACFT THAT WAS AHEAD; BELOW AND TO OUR L AND THEN HAVE US VISUALLY LAND ON RWY 21R. AT APPROX 6.5 DME WE TOLD THE CTLR THAT UNLESS WE COULD REDUCE OUR AIRSPD WE COULD NOT LAND. CTLR REPLIED BY HAVING US DISCONTINUE THE APCH. WE WERE VECTORED TO THE S THEN E IN ABCX PATTERN TO ALIGN OUR ACFT WITH RWY 28L. WE THEN WERE ALLOWED TO REDUCE OUR AIRSPD. WHILE HEADING E CTLR CALLED OUT TFC; A COMMUTER ACFT APCHING FROM THE E. WE DID NOT SIGHT THE COMMUTER ACFT AND INFORMED THE CTLR. THE CTLR THEN VECTORED US TO THE N AND THEN TO THE W FOR RWY 28L. WHILE IN THE TURN TO ALIGN ACFT WITH THE RWY THE COMMUTER ACFT WAS SPOTTED AT OUR 2 O'CLOCK POS AND ABOVE OUR ALT IN THE TURN FOR RWY 28R. WE INFORMED THE CTLR THAT WE NOW HAD THE COMMUTER ACFT IN SIGHT AND THAT IT WAS ABOVE US AND BEHIND US. CTLR RESPONDED ROGER; MAINTAIN VISUAL WITH THE COMMUTER ACFT AND SWITCH TO TWR FREQ. WE SWITCHED OVER TO SFO TWR CTL AND INFORMED THE CTLR THAT WE COULD NOT MAINTAIN VISUAL WITH AN ACFT THAT WAS ABOVE AND BEHIND US. THE TWR CTLR REPLIED THAT IT WAS NOT A PROB AND THAT WE WERE CLRED TO LAND. WE LANDED ON RWY 28L. I FEEL THAT APCH CTLS ACTIONS IN THIS PARTICULAR EVENT WERE POTENTIALLY DANGEROUS. WE WERE NOT VECTORED TO A POS FROM WHICH A SAFE LNDG COULD BE MADE DURING APCH TO RWY 28R. APCH CTLS DIRECTION TO HAVE A FLC MAINTAIN VISUAL WITH AN ACFT THAT IS ABOVE AND BEHIND THEM WHILE ON FINAL APCH IS NOT ONLY IMPRACTICAL; BUT IMPOSSIBLE AND DANGEROUS IF ATTEMPTED. FURTHERMORE; APCH CTL'S CLRNCS SET THE STAGE FOR THE DEVELOPMENT OF A SERIOUS CONFLICT.