The Licence Agreement for Mountain Lion may be different in your jurisdiction however the UK Mac App Store states for the OS X Mountain Lion License Agreement:
B. Mac App Store License. If you obtained a license for the Apple
Software from the Mac App Store, then subject to the terms and
conditions... you are granted a limited non-transferable, no exclusive
(i) to download, install, use and run for personal, non-commercial
use, one copy of the the Apple software on each Apple-branded computer
running OS X Lion or OS X Snow Leopard ... that you own or control.
If you are a commercial entity, then different rules apply
Both machines need the Mac App store to be accessed using the same AppleID used to purchase Mountain Lion. You can also save yourself the 4GB download a second time around, by copying the Mountain lion Installer from the /Applications directory tot he other machine once downloaded in full, note the installer is deleted once it is installed so cancel the install once it is downloaded, copy it somewhere safe, then upgrade away.