Toilets, Sinks, Showers, Tubs and Disposals - Toilets behaving badly

Hubby's gone fishing ... He'll be pleased if I can get a headstart on identifying the problem before he gets home Sunday.

Situation: Two American Std circa 1970 toilets, back-to-back in separate bathrooms. Since late yesterday, when one is flushed, it fills with water from its tank normally, but it somehow also siphons the water from the other toilet's bowl in the process. When the one with too little water is flushed, the reverse process happens, and the other ends up with too low a bowl level.

All the discussions I've read about vent issues talk about "gurgles" and odors... Haven't noticed any of that. Just one bowl sucking from the other. Could it still be a blocked vent, or something else?

Hi midwest1,
Welcome to the DIY Forums.

Your problem does sound like a vent issue. Drains need air to operate properly. When a drain is used, it sucks air in through the vent pipe. If the vent is clogged, it will suck air from wherever possible. In your case, it's the other toilet. (closest supply) A vent acts kind of like sticking a drinking straw in a glass of liquid and placing your finger over the end of the straw and lifting it out of the glass. The liquid stays in the straw because it is "airlocked". If you release your finger, the liquid flows out of the straw. This is the prime time of year when vents get blocked. Common causes are birdsnests, spiderwebs with leaves on top of them etc. The vent needs to be cleared. This entails going onto the roof and scooping out all the debris you can by hand and then flushing it with a garden hose. Good luck.

Thanks for the welcome and the advice, majak. Your explanation is clear and makes sense. It gives me a starting point for fixing the problem that I wouldn't have been sure about otherwise.
I appreciate it!

May I ask a followup question?

I was talking with a neighbor who knows someone who had a similar issue. A plumber found that the wax ring had failed, and even though it wasn't leaking water, it was pulling air from it's neighboring toilet. Replacement of the ring solved the problem.
Is this possible? One of my toilets was just reseated sometime during the past 2 weeks after being uninstalled for remodeling.
Seems to me even more plausible than the blocked vent. What do you think?

While anything is possible,THIS is highly unlikely. Reason being, you have a 3 or 4 inch vent line, why would it suck air through a trap. I am still betting its a clogged vent line. This is my personal opinion.