I have been asked to make 6" and 8" cakes that are at least as tall as they are wide...up to 10" tall! I am a little concerned about stability. Has anyone done this? What tips can you give for avoiding disaster? (Normally my cakes are about 4" tall).

You'll be fine as long as you use supports every 4" or so, and use a center dowel if transporting.

yep!

Thanks for your replies. I had thought of a center dowel...but wasn't sure if that would be enough.

I believe those are called double barrel cakes and yes keep your layers supported. treat them as two separate cakes with cake board and dowels in between each 4 inch and then cover it all with buttercream or fondant. HTH