This was not made clear in the original text but will be updated for the supplement. Please consider the following text as being on page 77 of the D:VG rulebook:
Vehicle Builder: Assign Vehicle Weapon Choices (D:VG 77)
When arming vehicles before a game, players may purchase more than one weapon per weapon slot. This represents redundancy in the weapon system. Note that players may still only fire one weapon per slot per game turn, with the exception of CDWs, which do not require a fire action.
A Vobian player has some extra points left when building a force for gameplay. He decides that he will maximize the offensive capability of his Mini-Jet to include anti-infantry and anti-vehicle weaponry. The Mini-Jet has only one weapon slot, with the choices of Gigawatt Multilaser (+177) or Plasma Penetrator (+162). The player can spend 339 PV (!) to give his Mini-Jet the flexibility to fire either weapon on any given turn, but in no case may he fire both weapons in a single turn, since they each correspond to the same weapon slot.
If he had fewer points to work with, the same Vobian player could have instead purchased both a Gatling Laser (+62) and Single Use Mega-Plasma CDW (+36) for his Cyber-Rhino. Note that, in this case, the player could use both weapons in the same game turn if he chose, since CDWs do not require a fire action.