Read the following excerpt from a web article on how to do what you want to do:
After providing your Apple ID and password, Mountain Lion will begin downloading. Specifically, the 4.4GB installer application, called Install OS X Mountain Lion.app, will be saved to your main Applications folder (
/Applications) and, if you’re upgrading from Lion, added to Launchpad. On my Mac, the installer even automatically launched after it finished downloading. On cable-model connections around the U.S. the morning of Mountain Lion's release, the download took between 35 and 55 minutes for Macworld editors.
Note that unlike with Lion, which continued to use the legacy Software Update feature (found in the Apple Menu) to provide OS updates, updates to Mountain Lion will be provided through the Mac App Store app. In fact, if you choose Software Update from the Apple Menu under Mountain Lion, the Mac App Store app is launched.
Downloading the installer onto other Macs: Once you’ve purchased Mountain Lion, you can download the installer onto any 10.8-capable Mac authorized to use your Mac App Store account—just like applications purchased from the store, a single purchase of Mountain Lion lets you install it on all your personal Macs. To download the installer on a Mac other than the one on which you purchased it, you just launch the Mac App Store application, click the Purchases button in the toolbar, and click the Download button next to OS X Mountain Lion in the list.
Copying the Mountain Lion installer onto other Macs: Alternatively, once you’ve downloaded the Mountain Lion installer onto one computer, you can copy it—over your local network or by using a flash drive, DVD, or external hard drive—to your other Macs. This is obviously a much faster approach than re-downloading the 4.4GB installer onto each of your Macs. You won’t even be prompted to authorize the installer on each Mac, as you are with other Mac App Store-distributed software—the Mountain Lion installer does not use digital-rights management (DRM).
Hope this helps
Message was edited by: Radiation Mac
Have a read here...
From the Above Link...
...when installing, if you let it go all the way by itself, the actual installer will be removed when complete, meaning you’ll have to download it again if you have another Mac.