When bitcoins are transferred, all bitcoins have to be sent out. If you are sending only some of the bitcoins from one wallet to another wallet, then the remaining bitcoins are sent back to the original wallet. This is probably why you see two wallets on blockchain.info.

For example:

Wallet 1 : 100 Bitcoins

Transaction: Transfer 20 bitcoins from Wallet 1 to Wallet 2.

Blockchain transaction:

Wallet 2 -> 20 bitcoins
Wallet 1 -> 80 bitcoins