I think Eli did not perform a fusion shot in The Emperor Strikes Back. A fusion shot can only be possible with a fusion core powered by Vitalis Crystals. I think the Fandango just boost Burpy to become more powerful. Look at the Fandango page, one of its attacks is the Levelest, which significantly powers up another Slug when shot in tandem. In The Emperor Strikes Back, Eli shot Burpy and Charger in tandem, which was at the same time. Charger powered up Burpy, making him more powerful. Eli did not use the Double Barrel, so that was NOT a fusion shot, but a move of a Fandango.