So in the current Texas border example, while the surface and mineral rights may be the same, they would be severed if the landowner and fedgov agree to do so.
Trying to give clarity, not trying to nit-pick.
Surface and Mineral rights are not the same, but they may or may not be owned by the same person.
It is always easier if eminent domain proceedings are not required. If the federal government and the landowner agree, anything may be included in the sale, including adjacent land, mineral rights, etc.
If the landowner refuses, and the property can only be obtained by an eminent domain proceeding, the government is only going to take was it needed for the project, adjacent property, mineral rights will not be awarded.
So the property owner may get a better deal up front, selling land on the other side of the wall that becomes difficult to access. I see no reason to ever sell the mineral rights unless offered enough money. Little reason for the government to buy them.