aragorn49:

Basically,
1) Start one game
2) Press Start on 2nd controller
3) Select a Hero - This will give you a henchman based around the loaded Hero's abilities, Although he will not look exactly like your original hero,
4) Select the Gold/XP sharing
5) Play
6) Finish
7) You save, this will take your earned XP and Gold back to your actual hero.

Please note, There is a glitch, that if you take an adult character into a Child's game, The adult's character will have his abilites and XP and Gold wiped... So again, To clarify, ONLY join couch co-op if your both in the adult stage.

Hope this helps.
(also note, this is what I have gathered from reading... I do not yet have the game, so could be wrong )