It will be as hard as doing three A-levels as intensive courses, for two straight years. It's unnecessary, even for applying to Trinity College, Cambridge for a Mathematics BSc (A*A*A* at 95% + STEP 1,1 should be fine). The main problem is that you'll be stretched thinner than Rome's military in the height of its empire.
Mathematics and Further Mathematics are great facilitating subjects for almost any course, whether it's Computing or Medicine. The choice of Physics/Chemistry/Biology depends what you want to do as a degree.
If it leans towards the medical field, Biology/Chemistry/Maths is a good combination. If it leads towards computers, Maths/Further Maths/Physics is a good combination. If it's an Engineering course, 3 out of Maths/Further Maths/Physics/Chemistry is a good combination.