Back-to-back water closet plumbing arcana

Hello,

A few weeks ago I asked some plumbing questions, including how to plumb back-to-back water closets. The upshot was that UPC 704.2 (operative in my area) requires the use of a "double fixture fitting" (DFF), not a double sanitary tee or a double combo. Subsequently, two issues arose that have made me reconsider using a DFF. The first one is that the biggest ABS DFF I can find is 3", so I would have to reduce my closet drains to 3".

Second, I read the following article:

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claims that modern toilets develop enough flushing pressure that the DFF pattern does not adequately protect one closet from having its trap siphoned by flushing the other closet. The author suggests instead using a double combo or double wye, which I can find in 4". One thing the author doesn't mention is whether he is refering only to pressure-assisted toilets; I'm going to use the gravity Wellworth.

So, plumbing mavens, should I just go ahead with the 3" DFF, or should I try to talk my inspector into allowing the 4" double combo? I have plenty of altitude for using a double combo. Does the article above make any sense? It seems like, in addition to contradicting UPC

704.2, the double combo would also violate the "takeoff above weir" rule (UPC 905.5).

Thanks, Wayne

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It claims that modern toilets develop enough flushing pressure that

FWIW, I read the article and it's still murky.

I agree, the combo would seem to violate venting rules and would need a vent takeoff on each branch.

3" is the largest DFF in either PVC or ABS.

We had been using 3" Dbl San TEE with modern 1.6Gal toilets successfully. Yes, there is some disruption of trap water when the other is flushed, but I haven't seen enough to break the trap seal. From that, I have to imagine that the 3" DFF would perform better.

Hmmmm. Would it be worth mocking the connections up and experimenting? 2 of the toilets (bowl only) on sawhorses with some 3" pipe slapped into a DFF. Dump a bucket of water right into the bowl and watch what happens to the other one...

Jim