ACFT Y DSCNT THROUGH OCCUPIED ALT HAD LTSS FROM ACFT X. SYS ERROR.

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SEBOUND ENRTE ACFT X AT FL200; NEBOUND ACFT Y 270 DSNDING TO 210 WITH CONVERGING RTES. Y STATED AT CHK ON THAT HE WOULD HAVE TROUBLE MEETING THE RESTR ASSIGNED BY PREVIOUS CTLR. I TOLD ACFT Y PLAN ON WISKE (INTXN) AT 100; BUT FOR NOW DSND AT HIS DISCRETION WITH WISKE AT 100 FT IN MIND. Y READ BACK 'WISKE AT 100.' I REALIZED THAT I HAD PROBABLY BEEN MISUNDERSTOOD IN MY MUDDLED COM BUT DIVERTED MY ATTN TO FLT STRIPS AND A SPACING SIT WITH INTENT TO GET BACK TO Y. WHEN I LOOKED AGAIN I SAW FL207 IN Y'S DATA BLOCK AND INSTRUCTED HIM TO MAINTAIN 210. X AND Y WERE ABOUT 8 - 9 MI APART. I RETURNED TO MY SPACING SIT AS Y RESPONDED WITH ONLY HIS CALL SIGN. WHEN I LOOKED AGAIN SEPARATION WAS APPARENTLY LOST AND I ADVISED Y AGAIN THAT THE ALT TO MAINTAIN WAS FL210. HE ADVISED THAT HE THOUGHT HIS CLRNC WAS WISKE AT 100. Y RETURNED TO FL210 AND RPTED LEVEL. ACCORDING TO THE PRINTOUT; Y WAS BELOW THE 200 TFC BEFORE THE HORIZ DISTANCE DECREASED TO LESS THAN 5 MI AND IT WAS ACTUALLY MY CLRING HIM BACK TO 210 THAT CAUSED THE EVENTUAL LOSS OF SEPARATION. I SHOULD HAVE BEEN MORE CLR IN MY INITIAL COM WITH Y AND MADE SURE THAT I WAS UNDERSTOOD (MY PRIORITIES SHOULD HAVE BEEN WITH THIS SEPARATION/SIT). SUPPLEMENTAL INFO FROM ACN 269877: GIVEN XING RESTR FOR 30 S OF BELAIR (AIR) AT FL210. FMC WAS IN ERROR COMPUTING XING RESTR AND HAD TO TELL ATC TO AMEND CLRNC. ATC REMOVED XING RESTR AND AT SAME TIME TOLD US TO CROSS WISKE INTXN AT 10000 AND 250 KIAS. I READ BACK THAT CLRNC AND WE PROCEEDED TO DSND THROUGH FL210 TO CROSS WISKE INTXN. AT FL195 (APPROX). ATC CALLED BACK AND TOLD US TO REMAIN AT FL210; THAT OUR ALT WAS TOO LOW. WE PROCEEDED TO CLB BACK TO FL210 AND SAID THAT WE HEARD A XING CLRNC. ATC TOLD US THAT THAT WAS AN EXPECT CLRNC FOR WISKE INTXN AT 10000 FT/250 KTS. I KNOW I READ BACK THE CLRNC CORRECTLY; BUT IF ATC HAD ANY DOUBTS; HE DIDN'T SAY. I KNOW I HEARD A CLRNC TO CROSS WISKE AT 10000 FT/250 KIAS. THE EXPECT WORD WAS EITHER CUT OUT OR OMITTED.