SMT Y CLB TO OCCUPIED ALT HAD LTSS FROM ACR X. SYS ERROR.

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ACR X; DEPARTED MCI FOR DCA; AND SMT Y DEP MKC EBOUND TO STL. BOTH ACFT DEP MCI AIRSPACE APPROX THE SAME AREA. ACFT WERE PUT ON PARALLEL HDGS; AND I CALLED ZKC; SECTOR 42; FOR A HIGHER ALT FOR ACR X (I HAD PREVIOUSLY TRANSFERRED RADIO COM OF SMT Y TO ZKC; CLBING TO 15000 FT). ZKC; SECTOR 42; ISSUED ME AN ALT OF 23000 FT FOR ACR X. I ISSUED FL230 TO ACR X; TELLING HIM TO EXPEDITE THROUGH 16000 FT FOR TFC. AT THIS TIME; BOTH ARE CLBING OUT OF 14000 FT. ACR X TO 23000 FT; AND SMT Y TO WHAT I BELIEVED 15000 FT. UNKNOWN TO ME; ZKC HAD CLBED SMT Y TO 23000 FT; HIS REQUESTED ALT. WHEN ACR X WAS CLBING OUT OF 16000 FT; I OBSERVED SMT Y AT 15000 FT. I THEN TURNED ACR X TO A HDG (CONVERGING) TO JOIN THE SID; BELIEVING I HAD 1000 FT ALT SEPARATION; AND GROWING. LEAVING 16000 FT; ACR X SLOWED THE RATE OF CLB AND SMT Y CLBED AT A BETTER THAN NORMAL RATE INSIDE ZKC AIRSPACE. ACR X AND SMT Y CONVERGED TO 3.5 NM AND 600 FT SEPARATION. HUMAN FACTORS: DURING COORD WITH ZKC; I HAD TOLD THE CTLR THAT ACR X WAS EBOUND TO 'GET AROUND' SMT Y. ZKC TOOK THAT TO MEAN I WAS GOING TO LEAVE HIM ON A EBOUND HDG; NOT TURN HIM TO JOIN THE SID; WHICH WE MUST DO ACCORDING TO OUR LOA; SO HE CONTINUED SMT Y CLB TO 23000 FT.