Imported 1099Bs are getting assigned the wrong person’s SSN on our joint return. How do I override this? Taxpayer, spouse, joint options are not available.

on a joint return, it's not going to matter because the SSN associated with the a/c is not reported to the IRS by Turbotax.  By default the first taxpayer listed on the return is listed on forms 8949  and Schedule D.

Be aware that any imported document can be overridden by going to the specific section. For 1099-INT or 1099-DIV go to Personal Income then select I'll choose what I work on which will show all the income sections. Go to Interest and Dividends and select the update button right to them. It will show the list of all 1099-INT or 1099-DIV depending on which option was selected. Once you select the one that needs to be fixed, under the section named "This interest belongs to..." select between the options: you, your spouse, or both.