You could always create an account on both PCs with the same username and password... and us that to execute shutdown.
That doesn't work as the full names for administrator are
PC-A\administrator and PC-B\administrator and there's no way of giving local admin rights to the "other" one.
Quick fix - go to www.sysinternals.com and download pstools. In there is a nifty lil util called PSEXEC. Actually, since you want to shutdown, there's PSSHUTDOWN too... along with a whole lot of other useful PS tools.