When doing butt joints I always put 3" tape on each side. However...I'm doing it to 4'X 8' sheets. I like the extra strength for all the moving and turning I subseguently do. Of course...after that there will be another layer of fiberglass over the entire boat...inside and outside. When doing it only on one side I just felt that I was always right on the edge of breaking the panel...though I can't really remember ever having done that. If you are doing a kit boat with the panels already cut it certainly might be ok to put the fiberglass only on one side because...as said before...you will eventually put more fiberglass over each side. Make your choice...throw the dice. Hope everything works out ok.