Upgrading a Mac from Leopard to Lion I've two Macs: one MacBook Pro Core 2 Duo with 10.5.8 (Leopard) and the other MacBook Pro Core 2 Duo with 10.6.8 (Snow Leopard).

How can I upgrade the Leopard Mac to Mountain Lion while leaving the Snow Leopard Mac as it is for now.
Most of what you need is in this Ars Technica article.

Essentially:


Purchase and download Mountain Lion on your 10.6 Snow Leopard Mac. 
Follow the instructions from the article (or elsewhere here on Ask Different) to make the bootable disk.
Put the disk into your Leopard Mac and upgrade! (You may have to do a clean install.)