Question: Is there a Hadith that when two Muslims meet each other and they shake hands, and then recite Durood upon Nabi, before they separate, all of their (minor) sins are pardoned? And is it saheeh?
Answer: I am not aware of any hadith that mentions saying "allahumma salli 3alaa Muhammad". But there are hadiths mentioning "shaking" hands (the "shaking" is not mentioned, just letting the surface of the hands touch). Some of them mention saying that they should ask for forgiveness from sins and praise Allah, and that sins will be forgiven. It does not say all sins. These hadiths should be at least hasan, since they are mentioed in Fath al-Baariy without comment.
... and Allah knows best.