All amounts on a joint tax return are combined totals. The tax is calculated on your combined income, not separately for each spouse. The tax cannot be attributed to one spouse or the other. You didn't each pay part of the tax. The two of you together paid the total tax.
You didn't say why you want to determine how much tax each of you paid. What are you trying to do? If it's just an exercise out of curiosity, you can divide up the tax in any way that you both agree on. For example, you could divide up the total tax in proportion to your individual incomes. But if you have any joint income, such as from jointly owned investments, you would have to decide how to handle that.
