My parents have a house built in the 1800's. Built as a four-square, the house has fireplace, up and down, at both ends of the house. The fireplaces are situated above each other, but slightly offset, so they have independent vent flue's but within the same chimney. They have a ventless NG firelog set in the original firebox in the downstairs and now want to do the same for the fireplace just above it. Will they be able to have the gas line run up, through the flue and vent from the downstairs unit to the upstairs, or will there be too much heat generated for that to be safe? Basically they want to run the line up one chimney, above the gas logs, then drill through and stub out for the upstairs unit...