You couldn't do a joint-honours course? At Exeter, for example, there is something called 'Flexible Combined Honours' which essentially allows you to mix in anything you wanted. I would suggest that philosophy is a more serious subject than North American Studies, so bias or priority should be given to that.
Philosophy is the thinking about thinking and ideas. It's a logical subject and sets the foundation for all other subjects. For example, epistemology provides the foundation for knowledge and discusses the fine line between knowledge and belief. This is obviously relevant to sciences but also it's relevant to all other academic subjects. Each subject requires you to think critically about ideas and this is why people who study postgraduate at a high level, taking original angles and evaluating axioms of a subject, do PhDs/MPhils, They are philosophers.