Best Spot weld cutting method
If you don't plan to save the panel, grind it at spot welds. Saves a TON of time over cutting individual welds.
Saving the panel, a pilot drill bit (Halifax's example) works along with a spotweld cutter. I've used Blair and HF cutters. Either work just fine. Drilling a 1/8" hole or marking a larger style punch mark is good to keep the pilot from walking. Once you get the feel and timing, cutting through one panel is a breeze without disturbing too much of the secondary panel.