It depends. There is no easy way to determine a formula for determining a refund split on a joint return. I suppose the best way is to determine who made the most money during the year. For example if you made $60,000 per year and your spouse made $40,000, then you may be entitled to 60% of the refund and your spouse is entitled to 40%. This may be a simple way of determining the refund split.
Also you can factor into the income on how much was tax was withheld during the year with the person that had more taxes withheld getting the bigger cut. So here you have two criteria to work with 1) income 2) taxes withheld.
Keep in mind, I just provided this for food for thought because there is no easy answer for this based on your circumstances.