Flexliner through a rigid liner?

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New Member
Jan 20, 2022
Nova Scotia
I have seen others ask about connecting a flex to a rigid of course, but I have what might be insane, not discussed, or simply a non issue.

Am I nuts for contemplating taking the top 10' off the masonry chimney, and running a flex liner THROUGH a slightly larger selkirk style chimney? I dont want to terminate the flex liner and have a connection in what is essentially living space (current attic, easily converted to living area) and I dont want to lose the masonry on the lower levels.

I bought the home a decade ago (time flies) and twice annually I need to do waterproofing which slows but never stops leaks. Simply put, the chimney is on the way out (I now presume the roof actually leaks into the chimney from the backwards flashing done on the back of it... but I still need to get to it).
As brief as possible:
1. runs in the home, not on an exterior wall
2. main level is double sided hearth I fell in love with, but could never wrestle to do what I wanted
3. there is now a woodstove/insert in the basement and on the main level, each in their own 12x12" flue via flexible stainless 5 and 6" liner respectively.
4. badly installed metal roofing complicate chimney flashing, as well as the fact the chimney is length ways across the slope of the roof line. It is also less than a foot away from a higher roof line (ridiculous design, impossible to reach)
5. chimney is now 45 years old masonry. repointed but not perfect (cant even get to the back of it... several awful roll on products used on the back).
6. in attic, chimney is actually NOT straight and moves 8" in 2 directions, presumably to get close to a roof line, but never does get to a logical place there
7. chimney should likely be higher than it is now (only 1' higher than the second, higher roof line)

I have a future reno planned where I will likely take over the attic space the chimney goes through. Id like to take out the aging parts exposed to the elements as well as the screwy courses changing direction in the attic, and run a straight pipe for each flue so I can properly flash the roofline.

Are there horror stories running one liner inside another?
Am I going to do something awful I cannot foresee?

Thinking 6" flex through a 7" selkirk style.
Tell me Im nuts. Im all ears.

Thank you for your time.
Exterior image because it sounds ridiculous, and is.