Yes, you can use the SheevaPlug as a NAS. No, you can't hook two different computers to one USB device. Does your media playback device support ethernet? If your Sheeva is playing it's role as a NAS, your media device simply has to look for SMB shares, or a UPnP media server.
Your best bet is to convert whatever machine you are serving content up with right now to XBMC (running on Ubuntu is easiest, there's prebuilt distro packages for it available with minimal googling). Then, you have two options:
Option A: The easy method. Serve up shares on your plug PC using NFS to your XBMC machine. I am currently doing this so I have come connectivity while I build
Option B: Use multiple plugs to form a true NAS. You can use open source tools to convert the multi-machine setup into a true NAS, although it's still a JBOD you will have access like a NAS. I'm working with FreeNAS at the moment and it's pretty slick. I tried OpenFiler, but it offers way more than I need -- FreeNAS is just fine.