You cannot weld closed vertices... that is, vertices which have faces enclosing it on all possible areas. You can weld two boxes together, if you've deleted the faces that would be on the interior of the weld, for example.
If you mean like a car then you can by using the symetry modifier it will automaitcally weld the two meshs together and gives you a threshold option to change the distance of the weld. hope i helped