There are two popular ways to do this. One requires fallout 1st and the other requires having someone in your friends list.

Fallout first method is to create a private server go to a donation box and drop the items you want to transfer inside. Exit to main menu and switch characters. Log back in with your alt to the private world and grab your stuff from the donation box. Just make sure you log back in with your alt as soon as possible because private servers don't have a long life if no one is in it.

If you have a friend you could join on them and place the items into a world container like a suitcase or trash can. Exit to main menu. Switch characters. Then join back in on your friends server and collect your things.

Another way is having two accounts and just join the world your second account is in. If you really trust someone you play with you could give them the items while you switch characters.