It's a neat trick but probably an unintentional bug. You can use a tinkertool to fix another by dragging it over the other one. By going back and forth like this you can max out their condition. I had a tinkertool at 200 and one at 191. When I dragged the 191 over the 200 it was fixed. I then had a tinkertool with 198 and one with 200, a net increase.
The ability to repair another tinker tool was pointed out 56 days ago but it didn't document the net increase.