You cannot form a sole proprietorship with any other person, spouse or otherwise. By definition, a sole proprietorship can have only one owner. As soon as more than one owner gets involved, the entity would have to become a general partnership. However, you should speak to an attorney and an accountant about the wisdom (or not) of doing business in a form that leaves your personal assets open and vulnerable in the event of a lawsuit or judgment -- which is the case with sole proprietorships and general partnerships. There are forms of business entities, such as S corporations or limited liability companies, that may be able to provide you with the tax advantages you are seeking without exposing yourself to personal liability.
Creating a MAP will take no more than an hour of your time every month and will keep the lines of communication open, ensuring relationships with investors remain strong, and ultimately helping early-stage startups succeed.