Turn switch to offline mode. Enter game, go to stash. Wait to see pentagram saving icon. Take items you want to double out of inventory, or personal stash, move them to shared stash. Hit home button, force close the game. When you rejoin there should be the items in your shared stash, as well as inventory or personal stash.
It took me a couple tries to master it, but it truly is simple. Obviously this only works offline