NMAC BETWEEN ACR-SMT ON STRAIGHT IN APCH AND LTT ON CIRCLING APCH FOR PARALLEL RWY.

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WE WERE CLRED FOR STRAIGHT IN VFR APCH TO RWY 26L ONT. ON CONTACT WITH ONT TWR; THE CTLR CALLED US TFC 2 O'CLOCK; 3 MI--MDT ON VISUAL APCH RWY 26R. WE SAW AND ACKNOWLEDGED THE TFC. ON OUR APCH TO RWY 26L F/O; WITH SHARP CONCERN; NOTIFIED ME THAT THE MDT WAS ON A COLLISION COURSE COMING RIGHT AT US. THE PLT OF MDT THEN MADE AN EVASIVE TURN TO THE RIGHT; WHILE WE WENT TO THE LEFT SHARPLY TO AVOID HIM. THE MDT THEN; AFTER AVOIDING US THE FIRST TIME; RETURNED BACK TO THE LEFT; RIGHT IN FRONT OF US MAKING US MAKE ANOTHER LEFT TURN WITH ME ANGRILY ASKING; 'WHAT'S THIS MDT DOING?' THE MDT THEN WENT BACK SHARP TO THE RIGHT; THEN PROCEEDED TO HIS CORRECT RWY OF 26R. BOTH ACFT LANDED ON THEIR RESPECTIVE RWYS W/O FURTHER INCIDENT. SUPPLEMENTAL INFO FROM ACN 84353: TWR ASKED; 'WHAT ARE YOU DOING?' TO THE MDT. HIS RESPONSE WAS VERY UNSURE OF HIMSELF. THE GUY WAS CLUELESS TO WHAT WAS HAPPENING!! AFTER LNDG THE 2 PLTS OF THE MLT ON THE RAMP WERE LAUGHING AND JOKING AROUND AS IF NOTHING EVER HAPPENED!