everything you need to know is in here: https://www.irs.gov/pub/irs-pdf/iw9.pdf
there are specific rules for what needs to appear on a substitute W9 (and a substitute form is anything other than an actual paper W9 form from the IRS), as well as rules to follow when you have a single signature and a dual signature like in a account signature card. For example:
If a single signature line is used for the required certifications and other provisions, the certifications must be highlighted, boxed, printed in bold-face type, or presented in some other manner that causes the language to stand out from all other information contained on the substitute form. Additionally, the following statement must be presented to stand out in the same manner as described above and must appear immediately above the single signature line:
“The Internal Revenue Service does not require your consent to any provision of this document other than the certifications required to avoid backup withholding.”
also - the signature does more than validate the TIN. There are four facets to it, and you need all of them.