Here are some leads. And you run a real risk separating these incomes.
Here are excerpts from the FTC site.
Franklin Acceptance Corporation "The complaint alleged that Franklin discriminated against applicants on the basis of marital status by failing to aggregate the incomes of unmarried co-applicants while aggregating the income of married co-applicants... Under the consent decree, defendants agreed to pay a civil penalty of $800,000 and to the entry of a permanent injunction."
Ford Motor Credit "The complaint alleged that the company failed to aggregate the incomes of unmarried co-applicants while aggregating the income of married co-applicants with the result that unmarried co-applicants often paid more for credit. Under the consent decree, defendants agreed to pay $650,000 to a consumer redress program and to the entry of a permanent injunction."