I believe SE earlier stated that the Mog Satchel would essentially function as a portable Mog Safe. If I'm imagining it correctly (and one never really knows with these folks), you can access your satchel from anywhere, but you can only use, equip, bazaar, and otherwise take advantage of items in your actual inventory. This seems to make sense, since the game could otherwise have some difficulty knowing which group of inventory slots to look at when showing you what you can equip, for example.
Hey, it's not exactly double inventory, but you can still get to that stuff at any time and just move it in and out of your inventory, so that's still pretty neat. I'm looking forward to having more space to carry random crafting materials.




