iPods do not have "accounts." They are associated (synced) with an iTunes library. One iTunes library can be associated with more than one iPod (as many as you want). You can set up each iPod to sync with the iTunes library separatedly, so each iPod can have different content from that one iTunes library.
So, you'll need to clarify what you mean when you say "each have separate accounts on one laptop." What is an "account" in this case...? Your problem is probably not directly related to the number of iPods. If your laptop's built-in storage is getting too full, the problem is that your iTunes library is becoming to large to fit.
You can tell iTunes to store its iTunes media (such as song and video files) on external storage. By default, the media files are stored in the iTunes Media folder, which is located in the user account home folder, which is on the laptops internal storage (where the Windows OS and app files are stored).
This document explains how to tell iTunes to use an external drive for its storage
http://support.apple.com/kb/HT1364
You are (1) telling iTunes the new location for the iTunes Media folder (on the extermal drive). Then, (2) you are consolidating your iTunes media files from its current location to that new location. And after confirming everything is working properly, (3) deleting the old iTunes Media folder to free up space.
However, if you are having problems with storage space on the built-in drive, that probably means you have a large iTunes library. "Plug-in memory" (like a USB flash drive) is usually not that large, so your iTunes Media folder may not fit. You should probably use an external hard drive instead.
NOTE: If you put your iTunes Media folder on an external drive, that drive will need to be connected and turned on when using iTunes.