Approach Controller providing OJT described a loss of separation event when the developmental failed to initiate enough corrective action to assure separation.

Narrative

A BE58 was on vectors to L38 airport and in a slow descent down to 3;000 FT. A BE20 was on vectors; just flying around in our airspace burning fuel prior to landing and level at 5;000 FT. My developmental was a little distracted and started to take action when the airplanes were approximately 10 miles apart and on converging courses. He issued a turn to the BE20; 'turn left 30 degrees'; the BE58 was still slow descending when I told the BE20 to climb and maintain 6;000 FT and turn left heading 270. Traffic was called by the developmental; the BE58 eventually had the BE20 in sight and said 'King Air in sight no factor'. The proximity of the 2 aircraft at this point was 2 miles and 700 FT with the BE58 out of 4;900 FT descending to 3;000 FT and the BE20 out of 5;600 FT in a climb to 6;000 FT. The aircraft flight paths never crossed. Mode 'C' variance may have been applicable in this situation; but I'm not too sure. [I should have] stepped in much sooner and not let the developmental get himself in a bind like that.