Ignoring the virtualisation for the moment, it sounds as though all you want to do is run one server with SBS 2011 and have a second joined to the SBS domain. That's fine - the SBS 2011 Standard license gives you the right to install SBS on server A, and the premium add-on includes a copy of Server Standard 2008 R2 you can install on server B.

There's no need to purchase two instances of the premium add-on (and even if you wanted to add in a new server, you would just purchase a retail copy of Server Standard rather than buying a second copy of the premium addon).

Now to deal with the virtualisation aspect. I don't know all the ins and outs of Hyper-V, but my understanding is that the 1+1 rights let you install Hyper-V on server A, then install Windows on top of that.