ZID CTLR EXPERIENCED OPERROR AT 17000 FT WITH CLBING AND DSNDING ACFT WHILE PROVIDING OJT.

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I WAS PROVIDING RADAR OJT. ACFT #1 WAS INBOUND TO DAY VIA RID; THE TRAINEE HAD PREVIOUSLY DSNDED IT TO 170 FPM FROM FL240. ACFT #2 WAS DEPARTING CVG NBOUND TOWARD MIE. THE TRAINEE CLBED IT TO 16000 FT AND ISSUED DIRECT ROUTING TO PMM. IN ORDER TO COMPLY WITH DAY ATCT LOA; I ADVISED THE TRAINEE TO RAPIDLY DSND ACFT #1 TO 11000 FT AND TURN IT 15 DEGS L. THE TURN COULDN'T BE ISSUED RIGHT AWAY BECAUSE OF ORD TFC WHICH WAS BEING VECTORED FOR SPACING; BUT IT WAS GIVEN ONCE THAT TFC WAS CLRED. I ALSO ADVISED THE TRAINEE TO CLB ACFT #2 TO FL220 (REQUESTED ALT). I THEN ADVISED A 20 DEG L TURN FOR ACFT #2. WHEN CONFLICT ALERT ACTIVATED; I ADVISED AN ADDITIONAL 20 DEG L TURN WHICH WAS GIVEN. AEDEP ACTIVATED AT 4.5 MI AND 200 FT (AT ABOUT 16500 FT). IN THIS CASE; I CHOSE TO LET THE TRAINEE ISSUE THE CTL INSTRUCTIONS INSTEAD OF TAKING OVER AND DOING IT MYSELF. I FEEL THAT THE EXTRA TIME IT TOOK TO RELAY TO THE TRAINEE WAS THE DIFFERENCE BTWN THE ACFT BEING SEPARATED AND SEPARATION BEING LOST. IN THE FUTURE; I WILL ISSUE THOSE INSTRUCTIONS MYSELF.