A Highland Dirk is sharpened on one side only. If you look, you will see that the spines of many of them are file-worked for a significant portion of the blade and that only about the first 2" - 3" back from the point is sharpened on the spine that measures over 10"
As an aside, you may wish to look at the Dirks that Michael Tinker Pearce has made. He does absolutely beautiful work but he does them only on commission, I believe. If you order from him, please read his commentaries at the top of his forum about his problems. Thay will give you some idea of what to expect in a special commission from him.
A small sample of his work on dirks may be seen at: http://tinkerswords.com/dirks.html
But, do see the Forum and read what he and others have to say about the experience of ordering from him. The basic need is patience.
I would suggest reading the entries in "Tinker's Daily Thoughts and Announcements", "Swords", and "Wish Lists!" forums. I would note that most small custom smiths seem to have similar problems and I honor Tinker for being willling to be "up front" and honest about his problems. This is as compared to ArmArt in the Czech Republic (Read the groans and such over in SwordForum about that one.) and the fellow in St. John, Newfoundland who took lots of people's money and then vanished.