I want to put a 9-inch two layer cake on top of another 9-inch 2 layer cake, creating one tall cake.

I think I've figured out how to do it, but oh great cake masters: How do I cut it???

You can cut it like any other 9" layer cake... they just happen to be stacked upon one another. Make sure you use a cake board for every 4" of cake height.

Yep... you're going to do a layer of cake, layer of filling, then a layer of cake. Then I put a thin layer of frosting on the top, dowel the cake, and then put a cake board on top of the frosting. Then do it over again : )
That way when you cut into it, you're just going to be cutting into the top two layers until thats gone.

Yes, I did the same on an Uncle Sam hat cake. One thing I didn't do but will the next time I have a cake like that is put a circle (or square depending on the cake shape) of parchment paper on top of the cake before putting the board and second cake down.
I use only buttercream and I read here that if you do this, it helps keep the icing on top of the cake below and not on the bottom of the board.

Thank you all so much! Great suggestions!