It sounds like it is a "Sole Proprietorship" even though you share it with your spouse. If you file Schedule C, the IRS considers it as a Sole Proprietorship.
For the W-9, I suggest you use the social security number of the spouse who has the most participation in the business OR you can always file for an EIN (Employer Identification Number) for the business. You can get an EIN even if you have no employees.
If you live in a Community Property State, you both will need to claim half the income if you decide to file separately. It does not matter how you fill out the W-9.
HOWEVER something you might want to consider if you both participate in the business.
You have the option to file TWO schedule C's for the one business.
Why would you want to do this?
If you file two Schedule C's you can allocate income to the social security accounts of each spouse which WILL affect future Social Security eligibility for you each.
It would NOT change the fact that you would each be TAXED on half the total income.
If you wanted to do this, both Schedule C's could be on the same Married Filing Jointly return, or one on each Married Filing Separately return.
How you report the W-9 still would not matter, if for example, it reports your social security number, you wound simply allocate the spouses portion of income as an expense on your Schedule C and that same amount as income on the spouses Schedule C.
@Victor21
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