Rather than have the iPods sync with your whole library I'd set up a specific play list or group of playlists for each iPod. You can specify to only sync certain playlists to an iPod under the Music tab with the iPod connected.

That's how I do it anyway.

You've got two options.

Firstly you could create two login accounts on your PC / Mac, which will give each user a distinct library, and just ensure that each user just syncs when logged in as their own user. It can get messy if you want to share tracks between both, and you can get error messages when you accidentally sync one iPod with the other user's iTunes, but it works well enough.

Secondly one installation of iTunes can easily handle multiple iPods at once. You can either elect to manage each one's content manually, or just create one or more playlists and sync those.

http://www.sillybit.com/libra/

It costs something like a fiver to download, but it allows you to create completely separate i-tunes libraries. I've been running various different ipods off it. The only downside is that every time it creates a library you effectively have to create a copy of your music library, and it can fill up your hard drive if you don't keep an eye on it.