Unless you put the account into a partnership where you can divide the income as you like between the 2 of you (and is the correct way to handle this when you have non spouses on a joint account) then the path of least resistance is for the primary account holder whose SS# is on the tax forms should report the entire amount on their return. If they complete their return without that info and then add that broker's info and see the difference it makes to the bottom line of the return with and without that amount ... then they can figure out the % of that difference that belongs to you so you can "settle up" with the other person.
