You can Pickpocket a cure potion back (level 2 required) after giving one to a person in the quarantine area, or kill a cured person (with no loss of reputation) to get them to drop it. Alternately, if you have both cure potions, you can duplicate them the same way you can with any stackable items (at least in the disk version):
Drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.