Two toilets backflow problem

[sparkspark]

Hi,

I've got two toilets installed back to back either side of a stud wall, they are joined with a branch, which then flows to the stack and vent. When the further toilet is flushed, it causes the trap level in the near toilet to drop by half inch whilst the other completes the emptying process. When flush complete, some dirty water siphons back to the near pan. I was thinking as the problem is neg pressure, could I install an aav from the boss connection on the branch, taking it up above the highest point in the pan via 40mm pipe.

Any ideas gratefully received

 

[Ermintrude]

Think there is a baffled tee for this purpose

 

[Dotty]

Simple fix, install a couple of anti cross flow valves.

Anti Cross-Flow and Rodent Barrier Valve - Accessories - WC Connectors | McAlpine Plumbing Products

 

[sparkspark]

Thanks for the quick reply, I'll try and get one locally in Bristol Monday, cheers.

 

[Destroyer2813]

Same as croopie said used em myself stops any cross contamination

 

[lame plumber]

problem being they should never share the same T in the first place.