There are several joint bank accounts and a brokerage account.  The agreement was to split the income but it is all being reported under one social security number.  The banks say they can't split it.

The client whose SSN everything was reported in backs out the other person's share as nominee income. They issue a 1099 to the other person reporting their share to them.

Consider also whether they've formed a general partnership under state law, and a 1065 should be filed rather than using the nominee method.