Cable length dilemma!

I also have the "fireplace between the speakers" situation (albeit a fake gas thing) and both cables are the same length (10 feet or so) with the closer speaker cable only looping once and crossing itself only once…no hum, loss, inductance issues…no problemo…it's an AQ Type 8 solid core pair so it's spiraled internally to reject hum, and sounds great.

I suggst finding a different spot for the speakers.  Anything large placed between them will be greatly detrimental to the sound.  If its a real fire place with a flu...all your lows will go up that flu.  Bad placement.

Move the speakers cable length should always be the same.

Close the flue…and put something on the roof to retrieve the lows that escaped! There's an easy test to see if the fireplace is "detrimental" and that is to put a rug or something that absorbs sound over the fireplace and listen to the system. I should note that I have had cables of differing length in other settings and it makes pretty much ZERO difference (and virtually unmeasurable if you're using good cable)…certainly not an audible difference, and "things between the speakers" is not necessarily an issue as it depends on the system and other factors of speaker placement. In my case anyway, the speakers are out front of the fireplace (you should get most speakers out into the room as much as you can anyway without causing a space issue, or if possible move them out just when listening) enough to obviate any reflection from the front of the thing, and the little corners created by the fireplace load the back ports a little and provide a spot for one of my subs. Besides, I can look at the fireplace gas dancing on the ever disintegrating "logs" while listening…all good.