I have had plenty of appraisal orders that have ONE borrower on the order, but have two people listed on the sales contract, I simply contact my client and ASK, if they want both parties (buyers in sales contract) listed in the appraisal report as the "borrowers" about 80% of the time, maybe more, my client tells me to ONLY include the name on the appraisal order listed as the borrower in the appraisal report.
As others have mentioned on here already, a buyer is not always a borrower, just because a persons name is on the sales contract and is later listed on the tax records, doesn't mean they are listed on an actual loan of a property a.k.a. a borrower.