The speed at which you switch from the mid-reload Evil Smasher to another gun and then back is the key to getting the upgrade glitch to stack. Using my mouse wheel to switch between weapons was too slow, or otherwise inhibited the glitch from occurring. Instead, I needed to switch between weapons using the number keys and my left-hand.
I'm trying the double run (and have watched the WR run for it), but I'm having an issue. For some reason when I have infinite ammo my smash is random, but doesn't build. It'll stay around 100 indefinitely.
After playing around I've determined that one difference is when I don't have infinite ammo the gun reloads automatically when I switch to it (and stacks normally). When I have infinite ammo I have to reload manually (hit the reload key) every time I switch to it. Also, when doing this if I have a smash, every time I switch to the evil smasher I lose one stack (ex: I have 117, switch to smasher, don't reload, switch back, I have 100, i.e. I have lost one stack). I've tried copying the steps in the run but it doesn't seem to work for me and I don't want to to stack a bunch and then start TVHM, I'd rather stack on the go, (for speed). So stacking and then getting infinite ammo is (hopefully) no an option.