I had two objects and joined them… what I did was select a line of verts from one object and did “P” and selected “selection” then I selected a line of verts from the other object and did the same… separated it… Now I had two lines of verts and joined them… went into edit mode and tried to merge the two verts near each other in the complete line… I have done it in 2.79B when modeling a car many times… not getting it to work in 2.80