You are able to do this, unless you have added a guaranteed stop-loss order to your trade.
To enable this feature, you will need to deactivate 'Account Netting': go to 'User Preferences' (in the top right-hand side of your screen next to the power on/off symbol), choose 'Account Settings' and then un-tick the box which says 'Enable Account Netting for this Account'.
Please note that shorting certain instruments may not be permitted – check the 'Product Overview' for confirmation.