Toto Drakes pulling water from each other's bowls

We just installed two Toto Drakes that sit back to back in adjacent bathrooms. I'm experiencing the symptoms described below.

AND HERE'S ANOTHER WARNING: If you have an adjacent toilet which drains into a common waste pipe, you may wind up flushing both toilets at once! The Champion flushes so fast that it creates a vacuum in the adjacent bathroom which sucks the water out of the bowl. I think if you read the comments here, you will see that I am not the only person who dislikes this bowl. In my opinion, American Standard has a lemon on their hands.
R.A.C. Sun, 03 Apr 2005

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That review was part of my rationale for not buying the American Standard.

The flush does not pull all of the water from the nearby toilet's bowl, but enough that the next flush would appear to be "incomplete" at best. I have checked the rooftop vent for any clogs and have found none.

Does anyone have any thoughts or suggestions?

I would think it to be the plumbing.

I've seen plenty installed back to back without problems.
In fact most homes plumb the toilets back to back.

If you have a San Cross fitting, the water can skip across the fitting, pushing air and displacing water in the opposite bowl.

Most high performance toilets require a fixture type fitting.

Flushed
I've experienced your situation. Contact Toto. They are aware of the problem. It occurs when the wcs are plumbed using a 3' figure #5 or doulbe fixture type fitting. They recommend a double combo setup. Unfortunately, a double combo with 2 90s might have a longer spred then the 30" to 32" common in back to back setups.
Fortunately my problem was not over a slab. I was able to cap one side of the fig 5 and connect one of the wc's to the lavy riser, turning it into a wet vent. Lucky for me I ran it 2" rather than 1-1/2".
Scott

Thanks for the advice. I think I'll try talking to Toto.

Looks like you have the room to put in a double combo. Check with Toto first, but I think you can get out of this one with your pants still attached.

In Washington State they never would have allowed a sanitary cross for two toilets.

They do allow a double combo

A double fixture fitting

This is still not good enough for the new back to back toilets from American Standard, Kohler or TOTO.
You need a double wye.