I have a Toshiba Qosmio X305. It came with 1 HD installed. There was room and a connector for the 2nd HD, but no mounting bracket. I got the bracket for $5 from Toshiba Parts. No one else had or knew about the 2nd HD bracket.
Also depending on the laptop, you may have either a firewire or eSATA port. Those both would allow you to use a 2nd HD. External USB is also possible but much slower.
Check you laptop specs, see what is available from there.