In my new home, we have back-to-back bathrooms (wife & mine, and the kid), with back-to back toilets, which have a 3" stack between them. My toilet is a powerflush while the kids have a "normal" flush.
My problem is, when we flush the powerflush, it siphons some of the water out of the kids toilet! After 3 or 4 flushes in my bathroom (and the kids toilet is NOT flushed during that time), there will only be @ 3/4" water in the kids bowl!
Like I stated, there is a 3" stack between them, so I'm not sure how I can break this vacuum!
This house is a ranch, with a full basement below these toilets, and access to any pipes underneath.