When two people own a business, you must decide what type of business entity this is and register it with the IRS. It cannot be a sole proprietorship, which only allows for one owner. Setting up a business entity is to protect you in addition to determining how much of the income and losses are yours to claim. You can register your business with the IRS, but read the full answer below before you proceed.
If you have set up a partnership or S-corporation, then you only report your share of the income and losses on your tax return. Your partner's financial obligations do not impact your return, unless you are legally married.
Partners who are legally married who are both involved in the same business can file as a qualified joint venture, which is a partnership that does not require form 1065. This only works if you are filing a joint return. More information about a qualified joint venture is below.
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If you and your partner are legally married, then you can protect your refund from being claimed by child support in one of two ways.