If I understand you, you don't and can't.
A notary can only affirm what they personally see. If the bill of sale
requires that both signatures be notarized, that usually means one notary has to witness each of you signing the bill, and each of you presenting valid identification (usually a government-issued photo
ID or something similar) to them when you sign.
One notarized signature, then faxed out for a second notarized signature, would probably be legal
but I'm not sure that anyone would accept it because it would not be conventional. In theory, each signature can be confirmed but that requires two confirmations now instead of one. Essentially you would now have two notarized documents--not one--with the second being a "hybrid" document, partly original and partly fax copy.
The exact requirements vary by state, assuming you are dealing with one or more US states. Some require seals
, others do not. Some require notary stamps, others do not. You'll have to find out what is required by whoever you plan to submit the notarized document to, because in the end they can choose to accept or reject whatever they please. And in some cases (like major banks) they refuse to accept documents that they are required
to accept by state law.
Incidentally...since something like 1999 there have been federal laws requiring that a properly digitally signed electronic transmission
(i.e. email) be given the same legal status as a signed paper document. But that's actually even harder to do, between getting digital signatures and finding anyone who knows what they are.
Bottom line: Ask whoever you need to give that bill of sale