I dunno....I thought about a clog too, but if the unit flushes fine to the tank as you reported, then what clog could there be. When flushing to the tank, there's space and ability to absorb air pressure through the breather, but not so much in the Lectrasan. It sounds to me like your one toilet is using the other for a breather cuz the Lectrasan can't handle the sudden compression. Look at your manual and see if there is a breather tube for the Lectrasan, perhaps a tube or filter cartrage that is partially clogged.
"I'd rather be happy than dignified".