Check this out: http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/ (http://www.environ.ie/en/Publication...dingStandards/)
Doc J deals with flues and chimneys.
I have seen shared flues many times in older properties, where a stove in the kitchen was an after thought and a hole knocked into chimney stack. I would not recommend it. you may have draught problems and more importantly allow carbon monoixde to enter living space when stove is alight.
I think the only reason that older properties got away with it, is because they are well ventilated(draughty!) as opposed to well insulated modern homes