... greeting with both hands is more common between people who...

... greeting with both hands is more common between people who know each other well and are on friendly terms. Nevertheless, this more cordial variant of the handshake can also be used with long-term business partners - as long as it is done correctly.
For example, if you place your left hand on top of the handshake, you are confirming the signing of a contract or your relationship with each other. This gesture also conveys gratitude and assures your counterpart of your absolute commitment. Placing your left hand on your counterpart's upper arm has the same effect, but is more suitable for a familiar greeting...