to get feedback, of higher quality, and more complete, and faster, post a diagram or two, and keep refining them. Stick to one thread and post a link to that thread in all the other threads you start or post to.
I have back-to-back toilets. They are wall-mount with Geberit carriers. May not be much that is useful for you to know. To connect the two, a Master plumber used one Wye and one San-T, whic is permissible since venting comes down from above and vents the first one, the San-T, and then the Wye is emptying into an already vented stack. The only other optin I had was to buy a back-to-back fitting from Geberit for this, but that i repeat was for wall-mount rear-dischareg toilets, not necessarily relevant to your situation but still good to know.