I just graduated with a BA in Sociology, and I've been accepted to complete a second BA degree in Health and Aging. However, another offer at another school came up, and I want to do that one. My question is, would it be possible for me to attend this other school this year, then come back to McMaster in Fall 2016? Or does the second degree have to come immediately after the first degree?

I know it would be a lot of money, but let's just ignore that aspect for right now.

You should be asking the registrar this question.

Once you have done a first BA degree, you are pretty well allowed to take most any course that you want (except for limited registration programmes). I did a pass BA decades ago, and recently did an honours BA. Check with the academic advisors at the faculty that you wish to do the second BA in, just to make sure there are no little secrets about programme capacity in what you want to do.

It would be best to talk to the registrar, if you haven't paid or registered I am fairly certain they don't care what you do. You will need to re-apply to the programme if you want to return.