At Enrolment Day they help you decide which subjects to do in Autumn and Spring and you get to enrol in them. Having said that, you can change them after enrolment day; the subjects you lock in on that day are not set in stone. For many degrees you will just be doing core subjects for the first...
Yeah, you'll be fine. I am doing the same double degree combination you are doing (2010 will be my second year), and the majority of students are doing both BCM101 and BCM102. So you'll find that everyone will walk from one lecture to the other as a big group.