You're saying two fireplaces shared this one common flue? Let me get this straight:
common vented fireplaces
copper gas lines in the flue
tiles broken out
mis-aligned tiles
mortar missing/ eroded
gas line routed through chimney wall???

So, what was your recommendation or comment on this? Good catch.
Bob

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Keep the fire inside the fireplace!
IAFCI Regional Director & Certified Fireplace Inspector
Certified Master Hearth Professional & HPBA BoD