You can have multiple wallets in two data directories. So:
- copy your existing data directory to a new one.
https://en.bitcoin.it/wiki/Data_directory- delete the wallet.dat file in it
- run bitcoinqt/d with the datadir option pointing to the new directory and a different port:
bitcoin-qt -datadir=<dir> -port=8334
https://en.bitcoin.it/wiki/Running_Bitcoin- It will create a new wallet for which you should set a password.
- create a shortcut for the above command.
It would be even easier if you used a client other than bitcoin-qt. Electrum, multibit, armory all support multiple wallets.