iTunes Match not merging librarys? What's going on?

Having signed up to iTunes Match, when I try to add a second computer to it I get a message saying I can only match one library at a time. But the Apple support info says it will merge the librarys. What's going on here? Have Apple lied to me or am I doing something wrong?

The message reads "Only one library can be matched at a time. If you continue, the other iTunes Match session will be stopped and this computer's library will be matched instead."

That's not what I paid my £21.99 for!!!