I fitted Mr Tuffys to tyres on my bike for a 1997 crossing of the Nullarbor Plain, country where double-g thorns (goat head equivalents) are rampant. I cut the strips to fit, one end concave, the other convex to butt together, then a short length of duct tape to keep them together. They worked perfectly, and I wore out the rear tyre before it ever flatted. To me, the duct tape provides a continuing surface over the join; the join can expand and contract, but there is no edge abrasion on the tube.
Neither Mr Tuffys nor Kevlar strips are a total solution to punctures -- they do not prevent sidewall penetration, nor pinchflats.