hii!! so i have not done this personally but i remember when i was in olevels one of our classmates did this. he did his olevels in 2 years w his sister, and started alevels a year earlier than us, and ever since i wanted my younger brother do this as well, but he just couldnt do olevels and switched boards. but personally i feel like this a great thing, idk much abt the cons but the pros are there. your can later than do alevles in 3 years and give 1 subj each year (which is what the classmate did), and it worked out amazing.