Basically, you can accept the degree they have offered you at St. Andrews and when you start, take other modules in subjects desired to get the right credits to add this as part of your degree? Does this make sense?
If you decide to go to St. Andrews, you will have a meeting with your academic advisor and discuss the other modules you want to take in other subjects. If your desired subject is for example, IR, which is highly popular, you will be put into a ballot and whoever is drawn will be able to take this as a module in the first two years, gain credits, and then be allowed to do it at honours (the final two years of the degree). Your name will only be drawn into the ballot if you were not accepted into this course initially, and if the subject is very popular.
In St. Andrews, we take 3 modules every semester for the first two years, part of our sub-honours. Then at honours, we slim down the modules from the 3 (some people continue with 3 if the right modules were taken from the beginning) and this is our final undergraduate degree. STEM students usually only take 1 extra module at sub-honours (or ones in their original degree), as their subject is usually very high demand and lots of other modules in their primary degree to take. But humanity students usually take 3, and some people take random ones every semester just to get the credits they need to pass into honours and graduate in the one subject they want to.
Of course, every subject has different module requirements and you need a certain amount every year to pass into the next, so is not always straight forward, and it becomes more difficult to make amendments to your degree after the first year.
So, you still have the possibility to take both subjects if you want. Just email the university and if you decide to go you will decide on your modules in freshers week.
Hope this makes sense.