May i ask why you are using blockchain.info?
Why don't you just use bitcoind (which gives you full control) ? Especially when running a service, i wouldn't recommend blockchain.info at all..
How can I set the second password?
This 'feature' is called 'Double encryption' and can be found on the My wallet account details page.
Make sure to call
walletpassphrase(password, timeout) with your password and a timeout in seconds before calling other commands.
This information is included in the link you have posted. Read the 'Security' paragraph..