This feature is now available in SlicerMorph. It is called MergeMarkups. You want to use the MergeCurves tab. If your curves are meant to be continuous (e.g., green and pink pair), check the continuous option, which when merging will remove the first LM of the second (and all subsequent) curves selected. If curves are separate, but they belong the same specimen and you want to store them in the same file for analytical reasons, uncheck the continuous option (default setting).
GIve it a try, and let us know how it works for you.
