Can't see keychain passwords on another mac
A lot of processes will keep the keychain open, and may not realize that it has been replaced by another keychain.  For example, right now I have

sudo lsof login.keychain 
Password:
COMMAND     PID USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
CalendarA   500  ats  txt    REG    1,4   753764 14974132 login.keychain
ubd         949  ats  txt    REG    1,4   753764 14974132 login.keychain
Dashboard  8779  ats  txt    REG    1,4   753764 14974132 login.keychain
Safari    29631  ats  txt    REG    1,4   753764 14974132 login.keychain
NetAuthSy 29809  ats  txt    REG    1,4   753764 14974132 login.keychain
WebProces 29823  ats  txt    REG    1,4   753764 14974132 login.keychain


You may need to restart things when you replace the keychain, or make sure only to sync the keychain when logged out.  (Which won't work well with dropbox.)

You might want to investigate something else like Keychain2Go which knows about the contents of keychains and makes sure to sync in a fashion that works while other things have it open.