Back-to-Back wall hung toilets

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I am a realtor and amateur designer ... currently about to tear apart my 2nd floor for a complete remodel ... adding a 2nd bath with the toilet backing up to the existing bathroom's toilet. I have a question regarding wall-hung vs traditional regarding the stack/drain. Here's a summary ...
My 2-story 100+ year old home sits on a concrete slab. This stack runs down through the first floor living room floor and out to the main lateral. That stack runs up through the living room to the attic above the existing 2nd floor bath. I know traditionally I would have to use a T WYE drain connection for this configuration ... however, unfortunately going down, it runs right through a brick surround for my Franklin Stove chimney. We did discover that the 2nd floor/living room ceiling joists are 2 x 10s. But, if I had to drop into the Living Room ceiling, I would have to destroy the fireplace surround

So I am thinking about a WALL HUNG back-to-back installation so we are above the floor .... but how would it branch off the existing stack? If the second one has to be slightly offset from the first one is there an adapter connections to have it connect to the stack below the first connection? I believe if we do an exact back-to-back, we would have to frame out the new bath wall to accommodate the second toilet. I have a great plumber, but in this case, I will look for someone who has expertise in wall-hung toilets. The issue I have is putting my estimate together for my renovation loan, so I need to know if this second bath is doable without going into the floor to meet the stack. Any suggestions anyone has on this would be greatly appreciated!

Tuttles Revenge

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If you intend to use a modern in wall tank style wall hung toilet you have two options. either line them up perfectly and use the back to back fitting. Or space them sufficiently apart that you can install a vent fitting between the must upstream toilet before it connects to the next. Geberit makes the most variety of options for in wall tank carriers.

Well.. just thinking. If you build the walls sufficiently apart. Then you can place the carries offset.. just need space for the drains
Hey, wait a minute.

This is awkward, but...

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