Explosive Backup in Shower/Toilet in Small House & Clear Bubbles in Sink & Toilet of Main House
It is very common to see (and/or hear) all manner of bubbling and gurgling when you have a partial or complete main line sewer clog. Call a reputable drain cleaning service. You can safely assume that running a cable down the line from the smaller house will get to the clog, but let the plumber tell you that because they often "guarantee" to clear the clog. Make it a point to be home when they do it, so you can inspect any material they drag up out of there. They may also be able to let you see what is going on using a borescope camera.