ONE ACR FOLLOWING ANOTHER ACR GET TOO CLOSE RESULTING IN LTSS.

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ACR X WDB ARR FROM BANDS INTXN. HDG 270 FOR SBD. ACR Y ; MLG ARRIVING FROM BANDS DIRECT PETIS NDB. LNDG ONT. ACR X WAS 12 MI AHEAD OF ACR Y AND ROUGHLY 2500 FT BELOW. ACFT LNDG SBD MUST BE OVER PETIS INBOUND AT 3200. ACFT LNDG ONT MUST BE OVER PETIS AT OR ABOVE 4200. THIS IS REFERRED TO AS THE PETIS OVER AND UNDER. IN THIS CASE BOTH ACFT WERE DSNDING FOR THEIR RESPECTIVE APCHS. ACR Y WAS ISSUED ALTS 1000 FT ABOVE THOSE ASSIGNED TO ACR X. HOWEVER; AS EACH ACFT WAS BEING VECTORED IN AN EFFORT TO HAVE THEM BOTH OVER PETIS AT THE SAME TIME AND AT APPROPRIATE ALTS; ACR X'S RATE OF DSCNT SLOWED DOWN AND ACR Y RATE OF DSCNT INCREASED. AS A RESULT OF THIS ACR Y DSNDED BELOW ACR X. NEITHER ACFT HAD EACH OTHER IN SIGHT. A TFC ALERT WAS ISSUED TO BOTH ACFT. THE PETIS OVER/UNDER IS AN EVERYDAY OP AT ONTARIO. THIS PARTICULAR ERROR OCCURRED BECAUSE I FAILED TO ASSIGN ALTS TO ACR Y THAT WERE 1000 FT ABOVE ACR X'S ACTUAL ALT. THERE ARE SEVERAL DIFFERENT TYPES OF SEPARATION AFFORDED TO ATC. IN THE PETIS OVER/UNDER EVENTUALLY ONLY ONE TYPE CAN BE USED -- VERT. THE ONLY WAY THIS CAN BE PREVENTED IS TO ENSURE VERT SEPARATION AT ALL TIMES. AS A PERSONAL NOTE REGARDING TCASII; AT A CRITICAL POINT WHILE ISSUING TFC INFO TO ACR Y; HE SAID MY XMISSIONS WERE BLOCKED. IN THE BACKGND I COULD HEAR TCASII LOUD AND CLR. LATER; THE PLT TOLD ME THE VOLUME OF HIS TCASII WAS THE REASON FOR NOT HEARING ME. ALTHOUGH TCASII WAS NOT THE REASON FOR THIS ERROR; IT IS CLR THAT TCASII VOLUMES ARE WAY TOO LOUD.