Are Mastercoin addresses Bitcoin addresses? I am reading up on Mastercoin, and I must say, it is quite confusing. Am I correct in saying that your Bitcoin address is your Mastercoin Address? If so, I have one private key that contains two cryptocurrencies?
Yeah, that's right - Mastercoin uses the same address space as Bitcoin.

You can store BTC, MSC, TMSC (Test MSC), and soon many other crypto-currencies on the same address using The Master Protocol.