many welds that seem impossible to most are just done from one side with the inner piece connected to one elsctrode and a single point spot head presses and heats the point to make the weld.
I have done tons of these with my Lenco spot welders. that is probably the top line unit closest to the factory single electrode spotters..
worth the investment as there are always the best for duplicating the exact and i mean exact pattern of factory welds.
also a tip when doing double sided welds. you can run a flat 1/2" inner electrode so most of the melt is between the sheets and top and the back surface stays un deformed.but be sure to monitor and controll your current and timing.
most factory outer panels ran 4-5ooo amps for .003 seconds.
less current and longer is better for playing round with the final weld look.