This should be really simple.
Go into your address book and select all the contacts.
Right click and select export Vcard
Choose where you want to save them, i suggest a dedicated folder and on a flash drive.
Once done put the flash drive in the other machine and select the Vcards and drop them into address book.
THis should add them into the other machine without deleting the contacts already there, the only ones you might have a little issue with is if you have the same contacts or people with the same name.
Hope this helps