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The wording is kind of weird. It's not actually a separate account with a separate login or anything -- but you can add more sites in the second account, and you get to them by choosing the account from that dropdown.
I find that implementation rather annoying. If I had known I was going to have to create more than one account, I would have done so from the beginning and grouped the sites in different accounts in a logical fashion. But there doesn't seem to be any way to move or transfer a site from one account to another, so I'm stuck with a mish-mash of sites in the first account, and the beginnings of some organization in a second and third account.