When you are doing this, you don't want your two batting pieces to overlap. You want them to butt side to side. Then use the tape -- half the tape, to the center of the tape will be on one batting piece and then the second half of the tape will be on the second batting piece.
The same as when you sew them together. Basically, you would hand sew them together with a whip stitch, making sure that the ends (sides) of the batting only meet and do not overlap.
I'm thinking that you are overlapping the batting pieces to get the 'lump'. You only want the pieces to meet, not overlap.