Just plug them in and instruct iTunes to automatically sync only certain playlists to each one. It doesn't matter if you have 579823745987398759837985739745 different iPods, iPhones, and iPads. iTunes will see them all as being completely different.
They could do that but, unless completely different people are going to use each iDevice and they want to pick and choose what content is synced while maintaining two separate iTunes libraries, the whole switching users thing is a lot of work when initially setting it up. iTunes is capable of differentiating between multiple iDevices, it is all a matter of telling it what to sync (or manually syncing).
As above...I have two iPhones, two iPod touches, two iPads and an iPod Nano all being synced to the same library.
Each device has a different set of songs, apps, media on. iTunes doesn't struggle with that at all.
That's because iTunes automatically sets itself up to sync all content in its library to any iDevice it syncs with. What you need to do is create a series of playlists. Then instruct iTunes to sync certain playlists to certain devices. That will keep things separate and stop iTunes from trying to clone itself on each device. Trust me, it really isn't that hard.
Create the playlists in iTunes as already instructed. Connect the iPod then select it in iTunes Devices list on the left. Select the Music tab on the right main pane and set it to sync to selected playlists noting the other options available. Select the playlists you wish to sync. Now Add and remove tracks from the playlists to maintain the iPod. I suggest making playlist folders named for each iPod to help keep things a little more organized in iTunes. Repeat the same steps for the other iPod, as already mentioned, iTunes will remember both setups.