i am venting 2 gas fireplaces through 1 chase. there will be only 18-20 inches between the direct vent termination caps horizontally . one fireplace has to be power vented because of long run (47 ft) fireplace manual iffy ,doesnt show this situation. one clearance says i need 3 feet from mechanical inlet supply duct. power venter manual shows 2 fireplaces through ROOF with minimum 6 feet between caps. company i install for tells me i only need 16 inches. I couldnt be more confused. any help greatly appreciated thanks, jay