No, they do not stack. It is a party skill, meaning that a person with the highest skill assigns his or hers party the skill point count. Put in context, you (a party's leader) have 6 points of a skill and your companion has 5. Your skill point count is greater, so it prevails; a party of yours (the one you command) is assigned 6 points of looting. This is why it sometimes is better to focus on leader skills, such as leadership or prisoner management and use one's companions for party skills.