Any “disinterested person” (i.e. any person not a party to the Note) can be a witness to the signing of a Promissory Note.

It is preferable that there be two separate witnesses. These two witnesses can be the same for each person signing the Note; there is no need to have four different persons (two for each signator) sign as witnesses to a Promissory Note.

If the Note is not witnessed when signed, the Note is still a contract that is enforceable in a court of law. The witnesses simply provide authenticity as to the signers of the Note. But these witnesses are not absolutely required in all states. (Note, however, that if the matter of enforcing a Promissory Note reaches a Court for settlement, the existence of Witnesses to the Note’s signatures will certainly shorten the legal process.)

Complete details can be found here on Standard Legal’s Promissory Note legal forms software.