On xbox all you have to do is bring your main character to Sanctuary and then save and quit to the game’s main menu. Log in a second controller, have them join BL3 in splitscreen, and then log your main in.
Your main will load, and on the second controller you need to choose what class you want. Then, a level 1 character will appear in Sanctuary and have a 16 slot bag. You can trade money and loot between characters, and with the second account you can make additional characters as necessary to store legendaries and other loot.
Edit: corrected spelling but also wanted to point out that you can’t use this method to give money to brand new characters on your primary account right away. What I’m going to do is take one of my Mule characters on the second account through the game until they get to Sanctuary, then I can do the process in reverse to invite Level 1 toons on my main account up there and transfer stuff to them. Also lets me upgrade their ammo SDUs etc before getting into the main gameplay again as kind of a shortcut.