A few things to point out here.
- When starting a business, always, always, always obtain and utilize an EIN with that business. Never under any circumstances and with no exceptions use your SSN. Doing so is an invitation to having your identity stolen. With an EIN, if that gets abused it only affects the business, and not your entire personal financial life. Remember, the IRS is the ONLY entity that issues EINs. You can get it in about 10 minutes absolutely free at https://www.irs.gov/businesses/small-businesses-self-employed/apply-for-an-employer-identification-number-ein-online
Now, the EIN is tied to one specific and explicit SSN. That's it. Period. So if "WE" are starting a business (as opposed to "I") then one of you must have your SSN tied to the EIN. The IRS doesn't care which one of you either, since your business will be a partnership (at a minimum) that files IRS Form 1065 - Partnership Return each year.