I haven't been able to find anything out on this, so I'll ask it here. I purchased the DLC that gives me all of the items in the shop, but when you do that, it doesn't actually "purchase" those items. Basically, with the DLC, I can buy all the items in the store twice. This means that I can have 4 assault coins in my inventory at once. Is this allowed? I haven't seen anyone use this in any runs, so I figured it might be banned. If it is banned, can someone point me to where I can find that rule?

We never banned it and I would argue that it should be allowed, only because it's more optimal in NG+ to only have two assault coins for inventory purposes, as that plus the infinite rocket launcher will allow you to one shot pretty much any enemy in the game already.