When you say merge, I'm assuming you simply want to add together the value of the two curves at each point along them. If so then you can define the two curves as A and B and then just do:
where C is then your merged curve.
I have assumed here that both of the curves are represented by two arrays of the same length. If not then you might need to use interp to get them to the same size before adding together.
If you wanted to do something more fancy then I think we'll need some more information on what exactly you want to do.