As an experiment I did the Elder Robe Quest to see exactly how the dev's set it up.

Basically you start by talking to a girl by the sp at Dath Research who talks to you about your robes looking a bit worn, etc. So she ends up giving you a 6 part quest to do as a favor for her, in exchange she will supply you with a brand new robe of your choice.

Quest : Collecting 6 Ancient Relics which are located at 6 POI's on 6 different planets (Yavin, Talus, Endor, Lok, Tatooine & Dantooine). This is rather easy besides the travel time and requires little to no combat (depending if you're attacked by Krayt's at the Tat Relic).

After retreiving all 6 Ancient Relic's, return to the start woman (this is where it gets interesting). Once there, you must drag and drop your existing robe onto the start NPC then converse with her afterwards. She then gives you a choice of the Arbiter or Oppressor robes as reward.

So you can NOT get 2 robes, it is via exchange only (light for dark / dark for light, etc).

However... the quest is repeatable so if you change your mind you just have to talk to her again and collect the 6 Relics, etc etc.

I marked each Relic's WP if anyone needs them, but the relic's are not hard to find once you get to each POI (check radar).