Working on the version 45803.90 for sure

How to do :

Locate the player save folder in appdata (some reported it to be on LocalLow for me it's in local/tinkerlands).

Simply copy the savegame01.player and rename it savegame02.player

As it looks that the slot of the save is also located for some reason in the data of the savefile, launching the world with a dupe of save 1 on save 2, edit the inventory of save 1 only. So you can just put the resources in a chest and load back the world with save 2 who will be untouched.

Even if it don't update visually on the character selection. You can update the dupe save and load it right away without reloading the game.