LGT X HOLDING AT PLANT INTERSECTION CAME IN CLOSE PROX TO LGT Y AT 10000' APPROX 1000'HORIZ DISTANCE.

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HOLDING FOR APCH CLRNC TO ORD AT PLANT INTERSECTION IN IMC CONDITIONS; 10000' ALT. APCHING PLANT X AT XXXX (EFC) SO WAS EXPECTING VECTOR TO ORD. WE MOMENTARILY WENT FROM NO VISIBILITY TO APPROX 1/2 MI VIS. SAW A LGT. JUST AHEAD; LEVEL; ROLLING OUT ON OUR HEADING AND COURSE. APCH CTL SIMULTANEOUSLY INSTRUCTED US TO CLIMB TO 11000'. COMMENCED CLIMB. AT 10500' SUPERVISION CAME ON FREQ AND INSTRUCTED US TO RETURN TO 10000'. WE INQUIRED ABOUT ACFT AT 10000' AHEAD; WERE ADVISED HE HAD DESCENDED TO 9000'. RETURN TO 10000'. I BELIEVE THIS EVENT CAUSED BY A LAPSE IN COORDINATION AT ORD APCH CTL. WE INITIALLY CALLED UP ON 128.45 (NOT THE USUAL FREQ). NORMALLY ORD APCH USES 125.7 OR 119.0. JUST PRIOR TO THE INCIDENT WE WERE TRANSFERRED TO 125.7. I BELIEVE THAT THE 3 FREQ IN USE AT THE TIME WAS A CONTRIBUTING FACTOR TO THE INCIDENT AS WE WERE MONITORING ALL CLRNCS IN THE HOLDING PATTERN TO PRECLUDE JUST SUCH AN INCIDENT. WE NEVER HEARD THE OTHER ACFT OR HIS INSTRUCTION AT ANY TIME. JUST PRIOR TO OUR DESCENT CLRNC FROM 11K TO 10K THERE WAS A GARBLED XMISSION THAT WAS NEVER CLARIFIED. AFTER ARR AT ORD; A TELEPHONE CONVERSATION WITH THE APCH CONTROL SUPERVISOR REVEALED THAT APCH ADMITTED TO HAVING CLRED BOTH ACFT IN THE SAME AIRSPACE.