It is vital to get these things properly resolved "up front" and to not rely upon promises of future action - it will all end in tears otherwise. I would creat

It is vital to get these things properly resolved "up front" and to not rely upon promises of future action - it will all end in tears otherwise. I would create a new legal entity to own the IP and divide the shares as seems appropriate to the founders. The new entity can enter into a license agreement (exclusive or otherwise) with the pre-existing business.