It doesn't matter that you pay tuition for a different department. You're allowed to come back for a second degree in a different department and it typically only takes 1-2 years (for non-hons and hons, respectively), assuming you've planned things properly.
This is the link to info in the old course calendar:
http://registrar-old.mcmaster.ca/CAL...ent/pg130.html (I'm too lazy to go find it in the new one, but you should check to make sure stuff hasn't changed).
So, sit down and plan what courses you're going to take throughout your four years, incorporating as much of the bio requirements as possible, and THEN go see an academic advisor. They'll be able to help you a lot more if you've already tried to set up all of the requirements.