About your last sentence, does this mean that I don’t have to wait 4 years to apply for medicine and can apply during my third year? Also should I be worried that I’ll have to maintain my marks for both IT and science compared to a single degree where I’d only be worried about science?
No you need to have completed or completing a bachelor's degree during the year that you apply. So you can't apply in the penultimate year of your 4 year degree. However, some people enrol in double degrees but drop one of those degrees in 1 or 2 years making it a 3 year degree which then you can apply in that third year. If you aim to complete the 4 year double degree you can only apply in that 4th year.
It is just that they use the final three years of your double degree to calculate your grades.
As for your last question i don't know - you are still doing 4 subjects a semester. I guess focusing on one discipline might be easier than focusing on two.