When OS X Lion came out, we had a post showing you how to install Lion on more than one Mac. It was a pretty straight forward process that involved copying the installer before it was deleted.
The same is true for Mountain Lion 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.
This time around you could use the same process or another way is the little utility Lion DiskMaker. It will also help you to create a bootable DVD or USB stick.
So if you have more than one machine and don’t have unlimited bandwidth, try one of these approaches.
Praseela Feltenstein on 
James Wingo on 
Hello, I bought Mountain Lion, but the installer is not on my laptop anymore – how can I get Moutain Lion on my other mac devices (desktop, mobile devices?)
Thanks for your help!
Layana.
If you’ve completed the upgrade the installer won’t be there anymore. Which means you’ll actually have to download it again
The idea is to download it from the App Store and then make a copy of it out of your Applications folder _before_ you click to start the actual install process.