If you got the Snow Leopard upgrade on an install DVD, here is an excerpt of the Snow Leopard licensing agreement:
2. Permitted License Uses and Restrictions.
A. Single Use License
Subject to the terms and conditions of this License, unless you have purchased a Family Pack or Upgrade license for the Apple Software, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at a time. You agree not to install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so. This License does not allow the Apple Software to exist on more than one computer at a time, and you may not make the Apple Software available over a network where it could be used by multiple computers at the same time.
C. Leopard Upgrade Licenses
If you have purchased an Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer as long as that computer has a properly licensed copy of Mac OS X Leopard already installed on it. If you have purchased a Family Pack Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household (as defined above), are used by persons who occupy that same household, and each
such computer has a properly licensed copy of Mac OS X Leopard already installed on it. The Family Pack Upgrade for Mac OS X Leopard License does not extend to business or commercial users.
(Unlike the newer OS versions which are only available via download, OS versions which were distributed via install disks, can only be used on one computer at a time; so, as long as you will only be running it on Mac 2, you should be fine.)