I have seen different situations where if the unit comes from factory as wood and oil boiler it ok to put in same chimney, But what is the differents from that to an oil boiler and a wood boiler in same chimney but two units? If when the wood boiler is off,not running the air intake is shut off then it will not effect the draft on the oil boiler? Never are the two running at same time, and even if they did the chimney is big enough for both flues added together. Why would one be ok and other not??