No, you can use both on the same PC and even then same iTunes account and still have different tracks and playlists stored on each device. You can do this by choosing the "manually manage" option on the main screen for the iPhone or the iTouch (or both). This way, instead of automatically syncing all tracks onto both devices, you can pick and choose which playlists, albums, individual tracks and videos etc are added onto each respective device. I don't believe there's a low limit to the number of iPod-based devices per account, only authorised machines (PC or Mac) per account.
It's probably not a good idea to have them both connected to the PC at the same time though. Although I get the impression that's not what you meant.