Easiest way on Xbox is creating some accounts on the website, you only need some emails, which is easiest if you own some domains like I do, but is also no problem with Gmail etc.
I created some spare accounts (created but not yet added on the xbox itself) so if I need a new mule I simply add one more, login with a second controller and then select a character and it joins you in splitscreen where you actually are.
Then you trade with it and transfer ~7 million to it to max out bank and backpack and that’s it you just got a new mule.
Also creating an account for each has the great advantage of being able to simply mail them the stuff while on the road and then later login with each one and sort the stuff.
I created one for Classmods/Artifacts, Shields/Grenades and now already two for weapons, but the latest for weapons is still at 20% capacity so no need to activate another one for now.
You of course also simply could create characters for the same account and login as guest, but this has the disadvantage that you don’t get more bank space but only backspack and that you can’t send them mail while on the road.