Does the total sq footage for calculating home office deductions have to include both of my homes?

My husband and I file jointly but live in two separate rented apartments, one block away from each other. I have a home office in my apartment. Do I need to use the square footage of both homes as a base to calculate the % of costs that I can deduct for my home office? Or can I use only the square footage of the apartment that has the office in it?