Two Kohler toilets (low profile, one piece) are installed back to back (powder room and master bath), sharing the same waste/vent stack.

The master bath toilet is equipped with a Kohler electric flush system. When flushed, if creates a powerful suction that drains most of the water from the other toilet's bowl. You can watch it get sucked down almost instantaneously, leaving only a small amount of water coming back up into the lowest part of the bowl.

Other than removing the electric flush system or redoing the plumbing, is there anything else I might try? I'm not a plumber, but am fairly handy.

Thanks in advance for any reply.

Your vent is either too small or partially clogged and can't replace the air fast ebough to aviod siphoning. Try rodding the vent from the roof. Do you know the size of your vent?

Ballangerb said it: it appears to be a vent problem. You should have 2" vent that is shared by both toilets. It may be clogged, missing or undersized.

Pulled a dead robin out from one yesterday, 1 bird big problem

Good point: this is the time of the year when birds are nesting. Also, it can be rat or squirrel...

Your toilets are served by a common vent off the lavatories. Did the toilets ever work normally? If so you have a blocked vent. The solution would be to snake the lavatory vent from the roof. Put out enough cable to reach the base and 20 feet more. Good luck, Tom