There are several ways to do this.
1 move your clips up 1 layer and split the screen leaving a gap between the clips and add a colour matte of your choice on the bottom layer, this will show through and create your line.
2 create a colour matte and crop it to the size your require and place over the join in your clips.
3 create a DV resolution, transparent image file in an image editor and add your line, save it to a format that supports transparency like .PSD or .TIFF and import this to premiere and add to the top layer.
I hope this helps.