Yeah this is painful. I ended up doing an elaborate work around for this precisely because of all the issues around VirtualBox, Cygwin, authentication and the users the corresponding services run under.
A quick option for you to try, enable networking between the VMs themselves, so VBoxGuest1 can login via SSH to VBoxGuest2 directly and issue the corresponding shutdown command.
If that doesn't work, you can do what I did. I installed VBoxVMService, went to Component Services->Computers->My Computer->DCOM Config->VirtualBox Application properties and chose my Identity. Then I installed Bitvise SSH server (not Cygwin .. so if you really want Cygwin I wouldn't know what to do). Now everything works seamlessly. Might be a little long winded for what you want to do, but I would appreciate if someone else provides an answer as well.