AIR CANADA FLT 839 (BOEING 727, C-GYNE) WAS DEPG CHICAGO O'HARE INTL ARPT AS CESSNA 650 (CITATION), N20AT, WAS DEPG PALWAUKEE (PWK) ARPT, ABOUT 8 MI NORTH OF O'HARE. BOTH FLTS TKOF WITH IFR FLT PLANS. BFR THE CITATION TKOF, THE PWK TWR CTLR CTCD THE TRACON NORTH SATELLITE CTLR (NC) & OBTAINED CLNC FOR THE CITATION TO TAKEOFF ON RWY 16 WITH A PWK #1 DEPARTURE. FLT 839 TOOK OFF FM RWY 32R AT O'HARE & BGN A RGT TURN TO 020 DEG, CLBG TO 5000'. AT ABOUT THE SAME TIME, THE CITATION DEPD RWY 16 AT PWK & BGN A RGT TURN WHILE CLBG TO 3000'. THE 2 ACFT CONVERGED NRLY HEAD-ON & PASSED CLOSE TO EACH OTR. FLT 839 ESTD THEY PASSED WITH APRX 500' SEPN, THE CITATION CREW ESTD 100' SEPN. BOTH FLTS WERE IN CTC WITH TRACON; FLT 839 WAS IN CTC WITH THE EAST DEP CTLR (EC); THE CITATION CREW WERE IN CTC WITH NC. THE 2 CTLRS WERE WORKING SIDE-BY-SIDE, SHARING THE SAME RADAR SCOPE. THERE WAS AN AVAILABLE LIGHT TO WARN OF ACFT DEPG PWK, BUT IT WAS NOT BEING USED. WRITTEN PROCEDURES FOR USE OF THE LIGHT WERE NOT SPECIFIC. TRAFFIC WAS DESCRIBED AS MODERATE TO BUSY.