I have always accepted the rule that you can't mix single wall and double wall pipe. I am not trying to argue but why the universal rule? I am rethinking my basement installation and slightly stretched the limits to combustibles on the ceiling. I have come up with a simple and inexpensive solution of using two 45's instead of a 90 to surpass the inch or so that I need. One of my options was to use double wall and that got me thinking.