Wow.
The only person I know who did a double major was my Chemistry teacher's daughter.
She was a brilliant student and a topper in her school. She got into a Ivy League University in the US which hardly accepted any international student.
She was studying Chemical Engineering and Masters in Economics both at the same time - and she was doing great.
I think it all depends on your determination, capability and how well you are
focused.
Masha Allah, you did really great in your A-levels - Imo, you can do it!
If you can do A-levels then you can do anything else!
You'll do great, Insha Allah. Just give it yer best! (: