I had the same problem of keeping two different desktops on a single system.
It also created a problem of selecting Contacts for the calendar and vice versa (Birthdays, meetings, appointments, etc.).
After doing a search and trying various trial versions, I have personally settled down on using Chapura's Keycontacs. http://www.chapura.com/keycontacts....38cff 8c164d5b
The prime reason for this is the multiple contacts and the exact field mapping it provide. It is kind of emulation Outlook Contacts on your Palm OS device.
The only drawback it keeps its data in a differently database. So if you already have contacts in your original Contact Database that came with your device, their information is kept in the original database. If you install Keycontacts, it will import all the information from your desktop and will create a new databse for the newly imported information.
I have tried to delete the old database but when I synch my NX it puts all the contact back to the original database.