Per IRS Publication 17 Your Federal Income Tax, page 26: 

Joint Return Test

You generally can’t claim a married person as a dependent if he or she files a joint return.

Exception. You can claim an exemption for a person who files a joint return if that person and his or her spouse file the joint return only to claim a refund of income tax withheld or estimated tax paid.

Due to this rule, it is rare for a couple to file a joint return with only one of the two claimed as a dependent on another taxpayer’s return. To handle this situation, the taxpayer eligible for his/her own exemption would need to be the primary taxpayer on the return, and the checkbox on Line 6b would then be unchecked for the spouse. To do this in your TaxAct return once the taxpayer eligible for his/her own exemption is entered as the primary taxpayer (and the spouse as the secondary taxpayer):

1. From within your TaxAct return (Online or Desktop), click on the Federal tab. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
2. Click Basic Information to expand the category and then click Filing Status
3. TEMPORARILY select Married filing separately
4. Click Continue until you reach the screen titled Spouse Exemption
5. Check the box Spouse Exemption and then click the checkbox again to clear that checkbox
6. Click on the Federal tab. On smaller devices, click the menu icon in the upper left-hand corner, then select Federal
7. Click Basic Information to expand the category and then click Filing Status
8. Change the status back to Married filing joint and now the spouse exemption box will be cleared on Form 1040 and only one exemption will show on Lines 6a and 6b