The other way is a butt weld with backing. What this is simply is putting a strip of metal behind the seam that you are going to weld. This is very common where you have no access like in a rocker for instance. You make a strip of metal about an inch to an inch and a half or so and you fit it behind the panel with it hanging out half the width of the strip for your next panel to lay over it. You punch a few small holes along your seam to hold that strip with plug welds. You get the panels right up to within a 1/16" to an 1/8" apart laying over that backing. You clamp it in place and when welding you hit the wire down on that backing starting your arc there then with the weld coming up onto the the two panels.
It's very common to when doing this in collision repair to have your new panel that you are cutting to size and you have that piece you cut off to use as backing. So you have this backing that is already bent to the shape of the panel like a rocker. You simply cut it off perfectly so size so that the piece you are cutting off is your backing.
Here is one in aluminum.