Brendan,
Why not put the files on a shared drive? Open PM on the desktop and let it work and do the other things you need to do via a laptop with PM open on it to work on the files in the same folder.
Legal wise it would require two licences of PM since with one licence you may use PM on any number of computers
but not at the same time. A single licence is limited to one user and one computer at the same time.
But if this simultaneus work is that critical it still may be worth buying a second license and as the one year for upgrades passes you still have to buy a second licence to be able to upgrade the next version (which is usually adviceable).
The question is if you have one licence for an older version that you cannot upgrade any more, and you buy a new licence to be able to upgrade than does it make PM a two user licence for the newest version (which you may upgrade on all machines with the second buy)?
Sorry to make this topic this legal sided but I am really courious and it connects to the issue.