How to drain and vent back-to-back toilets

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Zfredri

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Good Day, Plumbing Gurus,

I am seeking advice on how to properly drain and vent two back-to-back toilets in a symmetrical bathroom layout under a 4" slab on grade foundation with down-turned footers.
This is new construction, I am owner/builder, and am new to plumbing and trying to research and learn as I go, so take it easy :)

Is it okay to separately wye-in the toilet drains into the main drain, with vent just upstream, like the Figure 1?
The 3" vent is behind both wyes, upstream, and my concern is this setup would be creating an undesired S-trap, suctioning air from the toilet traps.

Another concern is the distance is 40" between the two toilet drain centers, which would be 17" from finished 2x6 wall, which seems far, despite that I'd want some "wiggle room" tolerance in addition to the required 12.5". This extra space is created by the angle of the wye coming from the drain which requires forward space to clear the second wye which translates to space from the wall, depending on how rolled upwards the wyes are.

An alternative, seen in the blue CAD Figure 2, would be to tie both toilets to a double wye with a vent above.
But I have seen diagrams where the vertical double wye is prohibited and could still create an undesired suction effect and/ or stream convergences.

The layout of the bathrooms is in the hand-drawn image, with a side view at the top of the page and my original layout sketch at the bottom of the page.

Some more photos are attached from different view angles.
Thanks for any help that you can provide.
 

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Zfredri

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Thank you, Master Young, for your input.

I agreed with your point that the double wye strategy between toilets is better, and re-plumbed it yesterday. In the original layout I had been confused about the allowable usage cases between double wye, double san-tee, and double fixture fittings.

In this re-do I went with a double wye between toilets with street 45 elbows. In between the shower/tubs is a double fixture tee. Here is a photo of the system.

My main question at this point is if it’s okay for the top of the closet 90 elbow to be level with where the top of the slab will be. If there’s a slightly lower slab could I shave some off with an angle grinder to make the closet flange’s bottom surface flush with the concrete?
 

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