Yesterday I finished up the fireplace/chimney Internachi online course and they mentioned that I should inspect for (if I remember right) more than one chimney flue that exits the chimney at the same height. They didn’t say why it was a problem so I made a note to research it later but have found nothing on the subject so far. Did I misread something or is this really an issue? Why is this a problem? I assume for drafting issues.
Well, 50 views and no response so I must have misread something.
I’ve seen answer of same height and staggered height. Not sure which is correct. I’ve read staggered height stops one flue from sucking smoke from the “active” flue.
Curious of the correct answer myself now. I was taught same height.
-Carl
Can we get one of the multitudes of CMI’s here to help this man out, please?
Chimneys side by each is not a current occurence, so probably why not to many responses.
Here is one by Carson Dunlap.
Separate metal chimneys that are too close to one another may cause damage resulting in poor chimney performance or an unsafe chimney.
It might be possible that one chimney is causing a down draft in the other due to an open damper in the unoperational flue.
I have read that there should be at least 12" in height differential between two chimneys, but on the other hand, multiple flue chimneys all have the flues the same height.
So I guess we are all open to opinions as to what it should be. 
I took that test not to long ago, I assumed the 2 flues were in the same Chimney, and said same height.
I can see the 2 flues in 2 different chimneys being staggered though.
Thanks, Marcel!
-Carl
Not that I’m questioning you Marcel, but exactly what would prevent the upper chimney from sucking in flue gas from the lower chimney even if they are staggered?
Being staggered would actually be a worst-case condition in that event.
I have had chimney smoke pull down an adjacent stack but the building was in such a huge negative pressure that the plastic vapor barrier in the crawlspace looked like an air mattress!
Why do we design flue pipes in the event of such an adverse building construction problem?
Why do we put gas-fired appliances in the laundry room so they can back draft continuously?
There’s a whole lot more to worry about in flue construction than termination elevation Pete.