GT,
The dual post headrests were a factory option in 1968, albeit an extremely rare option for some reason. However, IMHO, outside of Marchals, they are one of the sharpest looking options you can add to your car, as well as being an additional safety feature.

imp:
I’ve only seen three or four 68 Mustangs with the correct headrests although I think Paul Newitt’s car has them. All 1968 Mustang seat frames and the top piece of seat foam were built to accommodate the necessary brackets and posts for the headrests. I have lots of pictures and diagrams if you want me to email them to you. I think the headrests may be a little more common on the 68 Cougars and as far as I know, they are the same.
The true dual post 68 headrests are on Ebay a couple of times a year. Figure on spending $350 for a beat up pair and up to $600 for a nice set if you can find them.

Almost every set I have seen will have broken chrome tubes that the posts slide into and the foam will have some degree of separation from the metal base on the bottom of the headrest. None of the pieces of the 68 headrest pieces are in reproduction at this time.
Good luck!