They will be ordained as bishops April 27.
Fifty-eight-year-old Deshotel, a Louisiana native, is currently vicar general of the diocese.
Seitz, a 56-year-old Wisconsin native, is the pastor of St. Rita in Dallas.
Both men moved to the area in 1972 to attend Holy Trinity Seminary.
They have served in numerous parishes in the nine-county diocese.
Dallas hasn't had an auxiliary bishop since 1969 and it's the first time the diocese has had two serving at the same time.