True headlocked stereo is not possible in the ambisonics audio format.
Ambisonics audio is a 3D audio sphere and headlocked (fixed) stereo sound does not fit into this.
This is simply impossible.
Headlocked mono is possible, that is the top channel only.
360pan suite offer to mix to the top channel using the position blur control.
More position blur (more mix to top) is more mono, more inhead/headlocked.
When you pan your stereo source hard left and right (-90 and +90) in the ambisonics sphere and you set the position blur to half way (somewhere between 0.40-0.60) you will find there is stereo sensation while you cannot really identify things turning when you look around in the 3D sphere.
If you move the position blur control more towards 1 you will find it will rotate less, but it will also be more mono. That is the trade off.