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Bitcoin addresses today correspond to one public key in your wallet.
Signatures enter the picture when you spend the bitcoins sent to an address; your private key is used to generate the right digital signature, proving that you actually have that key.
BIP 16/17 will enable bitcoin addresses that are associated with more than one public key. Your wallet will know both public keys, but will only know ONE of the private keys needed to spend the bitcoins (your phone or a "wallet protection service" will keep the other private key safe).
So when sending coins, your wallet will provide one signature for the private key that it knows, the other required signature must come from whatever device is holding the other private key.
The public keys aren't just strung together in a row, but are combined using a secure hashing algorithm (the same algorithm that is used to associate public keys with the bitcoin addresses we're all using today).