Hi, Thanks for your enquiry!
We do accept applications to multiple courses, it's particularly common where students apply to one of our single honours courses, and then also a joint honours which contains their "main" subject. It certainly wouldn't be detrimental to your application for either course.
I've spoken to the Admissions Officers for both the Business School (Economics/Management) and Politics. If you applied for multiple courses with us, they'd both be able to accept an additional personal statement to cover the second course you'd applied to. I'd recommend that you email them to confirm this before you submit your application.
Perhaps one way to refer to your interest in joint honours courses within your main personal statement, would be to include your interest in how Economics fits with other subject areas/types of business or industry/your future career plans?
I hope that helps,
Amanda