can I upgrade both of my macs and only buy the new OS once?

You can purchase Mountain Lion once and install it on each of your machines. You do not need to purchase separate copies. In order to save yourself from having to download the entire package for each machine. When you purchase it from the App Store, it will download into your Applications folder and you will be presented with the screen to start the installation. Do not start the installation at this point. Make a copy of the installer which is in your application folder first. You can then go ahead and continue the installation. The reason for doing this is that once the installer finishes, it automatically deletes the installer that you downloaded. If you make a separate copy, you can put it on an external or network drive so you don't need to download it for your other machine.

Yep, but after you download on the first machine, make this in order to make installation on second machine quick:

http://www.macworld.com/article/1167857/how_to_make_a_bootable_mountain_lion_ins tall_drive.html

Do this BEFORE you install on first machine.

Or you can just do this: http://support.apple.com/kb/HT4854

wjosten and I were just pointing out that if you save the installer the first time, you don't need to download again for each machine. That's not referenced in Apple's installation instructions.