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 'Account Settings' and then un-tick the box which says 'Enable Account Netting for this Account.'
From 16 November 2024, with Account Netting disabled, you will be charged margin in addition to fees (namely holding costs) on both sides of the trade.
Please note that shorting certain instruments may not be permitted - check the 'Product Overview' for confirmation.