Hi there!  I'll be making a 2 tier cake, the bottom being a 2-layer 10" square, and the top a 2-layer 6" circle.  I see that bubble tea straws are great for support, but I'm wondering if I also need cake circles under my top layer for added support.  Or... do I just set the 6" cake directly on top of the straws?  Thanks in advance for the helpful tips

You definitely need circles under the top layer.

Thank you!  How many circles should I use? 2 or 3 layers, or is 1 enough?

just one to keep the top  cake from collapsing into the bottom one.

http://www.wilton.com/cakes/tiered-cakes/stacked-tiered-cake-construction.cfm

Thanks to everyone... I appreciate the help!