You won't automatically be put in the faculty of humanities if you combine across social sciences and humanities. Certain combinations are claimed by either faculty so depending on your combo, you could be in humanities or social science.
I would suggest meeting with an academic advisor to discuss the double major. They're really helpful, they'lll print out a degree audit for you and show you all the requirements you'll need for combined honours. They'll also tell you whether you'll be in humanities or social sciences.
You can check out the requirements online by looking at the undergraduate course calender. http://registrar.mcmaster.ca /calenda...2008/index.htm
Click on the faculty of social sciences and the faculty of humanities and click on the department of each of the majors. Click on the combined honours in _____ and you'll find your requirements.
You'll only get 6 units of electives a year doing a double major.
What combo are you planning on doing?