Executive summary: My experience indicates they should be able to coexist.
Longer explanation:
I was trialing Aperture 3 for a while, so I didn't upgrade my existing library - I created the v3 library from the vault. However when I decided to upgrade I completely removed that trial version of Aperture (by looking at the "Files" listing in the installer and deleting everything listed), then installed from a retail box - so that's the version I'm running, not the download.
For the moment I have separate libraries named "Aperture 2 Library" and "Aperture 3 Library", and separate apps "Aperture 2" and Aperture (which is version 3). Both versions of Aperture seem to launch and run just fine. When I launched Aperture 2, I did get a registration reminder, which is a bit odd; but after I clicked "Later" the app ran just fine.
Note that because of how library management has changed, once you've run Aperture 2 its library is considered the default - so if you then launch the Aperture 3 app, you'll get a warning about needing to upgrade the library. Fortunately you can quit without doing anything (so the v2 library won't be touched); but you'll need to select and launch v3 by selecting the v3 library that first time after running v2. Aperture 3 will remember this choice in the future, at least until you've run Aperture 2 again.