Depend what you're after. If you want to access joint files from either machine regardless of when the other is turned off, then you need a separate network drive such as the Linksys
EFG80 or
EFG120. These are dedicated devices with many additional features.
The other option, as suggested, is to use another old machine as a shared drive. You can pick up a suitable complete machine for around £99. e.g. see
here on ebay. You can set it up using the monitor from one of your existing machines then just leave it running in a cupboard or under a table.