From command line
shutdown /m IP-OR-MACHINE-NAME /t 00
Helps if you use the same account with admin priviledges on both machines or it's hard to do. So if you have the same admin account with same password doing the above will work.
I'm having problems getting this to work. It keeps telling me "access is denied". I use the exact same account name and password on both machines, both are "administrator" on the user account screen. One machine is Vista Home Premium, the other is Windows 7 Home. I can set up network shares and the like but it just doesn't want to accept the shutdown command.
Are you running an elevated command line?
That is searching for cmd in start search, then right clicking and choosing run as administrator?