If it's the preferences of plus ur talkin about, u can lock em:
Plus > preferences > privacy protection > passwords
enable "Enable preferences lock-down" and set a password
Suggestion: U can both get a messenger (.NET) account an run em at the same time (polygamy) if u use jrnzloader available here:
http://kaonet.net/list.php?c=msn
Pick out the jrnzloader for ur version in the list u see there