Synchronising contacts for different phones on same computer

I would appreciate any help.

I am using Windows XP and have two different iPhones registered on that computer. Last time I synchronised my wife's iPhone contacts it resulted in my contacts transferred to her phone although it clearly shows her name before synchronisation. In general I do not understand how the synchronisation works. Does the synchronisation have a "master" and "slave" relation? Is it the Outlook contacts synchronising to the iPhone or other way round. If I am away and add a contact to my iPhone will this contact add to my Outlook when synchronising? Thank you.