How to find union of two SURFPOINTS variables?

I have SURFPOINTS variables

surf1=detectSURFFeatures(Img1);

surf2=detectSURFFeatures(Img2);

How to find UNION of the two surfpoints surf1 & surf2 so that i get an object which is contains all surf features from both images and no repetition of features occur.

The straight forward union function is not working in case of surfpoints

surf3=union(surf1,surf2)

is throwing up error that sort feature is not aplicable for surfpoints object

Answer 1

[Qu Cao]

surf3 = union(surf1, surf2, 'rows');