I need to create a topology where 1 and 2 touch but don't overlap, a and b touch but don't overlap, and 1 covers a and 2 covers b. Is it possible to achieve this? I've tried various snapping options, but they all seem to result in one of the inner polygons overlapping with a wrong outer polygon. I'm using QGIS.
I seem to have found a solution that's working at least so far. I'm creating two vertices on the shared boundary of the outer polygons and then snapping the inner polygons to that.