Hi, I do this from time to time with a combination of external firewire LaCie burner (recently replaced the drive but kept the case, Firewire logic etc.) and Roxio Toast 7.
One of the options is to burn from an image and that would apply here. When setting up for the burn, you choose the disc type: CD-R, DVD-R or DVD-R/DL. Everything else just happens.
Very good combination: easy to use, has all the features I could think of and some I will never use and burns discs reliably (although I probably haven't done more than 10 DL).
I would suspect you could also do this from OSX specific tools, I haven't tried and can;t tell you how.
Ferdi