What's the legal status of my Snow Leopard once I've upgraded to Mountain Lion?
Yes, for the first question. Whenever you sell your Mac, you're expected to sell it with the OS it came with, which in this case is Mac OS X 10.6 Snow Leopard. Your upgrade to OS X Mountain Lion purchased from the Mac App Store belongs to you for your use and cannot be transferred to another person (on the other hand, if you bought a Mac with OS X Mountain Lion preinstalled, you're allowed to do a one time transfer to another person).
From section 3. Transfer in the SOFTWARE LICENSE AGREEMENT FOR OS X MOUNTAIN LION (emphasis mine):

A. If you obtained the Apple Software preinstalled on Apple-branded hardware, you may make a one-time permanent transfer of all of your license rights to the Apple Software (in its original form as provided by Apple) to another party, provided that: (i) the Apple Software is transferred together with your Apple-branded hardware; (ii) the transfer must include all of the Apple Software, including all its component parts, printed materials and this License; (iii) you do not retain any copies of the Apple Software, full or partial, including copies stored on a computer or other storage device; and (iv) the party receiving the Apple Software accepts the terms and conditions of this License. For purposes of this License, if Apple provides an update (e.g., version 10.8 to 10.8.1) to the Apple Software, the update is considered part of the Apple Software and may not be transferred separately from the pre-update version of the Apple Software.
B. If you obtained your license to the Apple Software from the Mac App Store, it is not transferable. If you sell your Apple-branded hardware to a third party, you must remove the Apple Software from the Apple-branded hardware before doing so, and you may restore your system to the version of the Apple operating system software that originally came with your Apple hardware (the “Original Apple OS”) and permanently transfer the Original Apple OS together with your Apple hardware, provided that: (i) the transfer must include all of the Original Apple OS, including all its component parts, printed materials and its license; (ii) you do not retain any copies of the Original Apple OS, full or partial, including copies stored on a computer or other storage device; and (iii) the party receiving the Original Apple OS reads and agrees to accept the terms and conditions of the Original Apple OS license.


For the second question, yes, you can install Mac OS X 10.6 Snow Leopard on one other Mac that you have. Look at the SOFTWARE LICENSE AGREEMENT FOR MAC OS X, where section 2. Permitted License Uses and Restrictions states (emphasis mine):

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.

You can do a one time permanent transfer of the Mac OS X Snow Leopard license. Section 3. Transfer in the agreement states (emphasis mine):

You may not rent, lease, lend, sell, redistribute or sublicense the Apple Software.
Subject to the restrictions set forth below, you may, however, make a one-time permanent transfer of all of your license rights to the Apple Software (in its original form as provided by Apple) to another party, provided that: (a) the transfer must include all of the Apple Software, including all its component parts (excluding Apple Boot ROM code and firmware), original media, printed materials and this License; (b) you do not retain any copies of the Apple Software, full or partial, including copies stored on a computer or other storage device; and (c) the party receiving the Apple Software reads and agrees to accept the terms and conditions of this License. You may not rent, lease, lend, redistribute, sublicense or transfer any Apple Software that has been modified or replaced under Section 2H above. All components of the Apple Software are provided as part of a bundle and may not be separated from the bundle and distributed as standalone applications. Apple Software provided with a particular Apple-branded hardware product may not run on other models of Apple-branded hardware.