Hey, just finished re-reading the book again and i noticed something referring to this (unless i was reading it wrong which is highly possible, im tired soooo...)
Anyways, right as joel finishes the melee when he is still drawing after the bell has rung he was working on making a "secondary bastion of defense because their main circle had been breached a dozen times" (<--- that was paraphrased but still gets the point across) however after typing this i am realizing that the bastion might have been the smaller circle inside, but that also works for your theory. Also if you look at the taylor defense sketch it is a circle inside a circle, and that is why its called the impossible defense (that and the fact that it is a huge 9 pointer) so to answer your question i would say that yes, you can use circles inside of each other, it just isnt done often because it is impractical? when drawing rithmatists kneel. If you were to draw two defenses inside each other i think it would become to large and harder to move around the the kneeling position as well chull take too much time to draw. You would be attacked before you could finish. Plus the size would need to be even larger because without drawing the binding circles on the second inner circle would take up more space, and if you left those off the lines stability would be thrown off. Hope that made sense, if not oh well.