You will need to provide a good bit more information before anyone can provide a meaningful answer. For example, how is the business organized (corporation, partnership, LLC).

In any event, taking your question at face value, it sounds like some of the ownership is legally in the name of the husband and some is legally in the name of the wife. That would suggest that you will have to pay each of them for their respective share in the business. Of course, there are many things that could impact this, one example would be ownership transfer provisions contained in a shareholder agreement or an operating agreement. It seems to me that you will need to have an attorney review the documentation for the business and then advise you.

What documentation is there as to the ownership of that half? Notwithstanding the unwritten agreement between husband and wife, you can only deal with the recorded owner, as that is the only one that can sell the interest. There may be claims between the husband and wife, but that is not your concern.