Sub majors don't appear on your official graduation certificate though. ALso in combined degrees rarely do you do a sub-major, the other units are filled by the other course.
How did not knowing where to go have any bearing on your original question, which was "Why are they on the same day". Even after people told you it was so you could go to both of them in the same day, you still questioned as to why they both weren't on the same day.
Conclusion: you are a...
Then why would you want to make a 6 hour train trip multiple times? And how would it make it any easier to reach the unis via public transport if they were on seperate days? This is why it's smarter to have some close unis on the same day.
There is no point doing a broader degree if you hate or suck at the additional areas it encompasses, such as statistical analysis and consumer analysation.