While common on older construction, it sounds like you do not have the preferred fitting between the two toilets. That can become a major problem if you decide to upgrade them to modern, high-performance ones, as it sounds like the fitting you have would allow the flush of one toilet to upset the one opposite it. The proper fitting will have different angles on the inlets...they're bigger, so harder to install without prior planning. That fitting worked with older, slower flushing toilets. Newer ones need to do more with less water, so they are designed to accelerate the water more so that they can clear the bowl. Older ones just used lots more water, the height in the bowl rose, then had its own head pressure to start to move things down the drain.