They were bred into the Fallen species ions ago, due to their deep hatred of seatbelts. Many, many moons ago there was a Fallen Emperor named "Cashtian Thalago III". He was a very powerful ruler who was respected and loved by all. In a very unfortunate accident involving a space craft, a small asteroid, a fat Cabal woman (don't ask) and a discarded "No Land Beyond" sniper rifle (I won't go into the details), he was trapped inside his space craft and died in a violent blaze. After an investigation that lasted many months, it was determined that the seatbelt had malfunctioned and he had not been able to free himself and escape his fire engulfed prison. At that point Fallen scientist began genetic research to prevent this sort of tragedy from ever happening again. They worked and worked for years at genetically engineering a suitable fix and finally were able to alter their own species to have a secondary set of arms whose sole purpose was to hold their butts into the seats of a space craft....

They have to earn the right to grow their arms. Dregs have docking caps to stop them growing vandal arms etc

Other two hands on captains is for jerking off and punching people.