Oculus doesn't really have a concept of primary hand. There is just a left and a right, and you can use them however you want.
The confusion may come from the fact that OVRInput has variable names like PrimaryHandButton and SecondaryIndexTrigger. In case someone else runs into this issue, know that you can check OVRInput.cs to see where those are set. If you want to check the input to a specific touch controller (or any type of oculus controller for that matter) you can use OVRInput.RawButton, OVRInput.RawTouch, ...