When getting bonuses for having Dark or Light Mastery, the bonus you receive depends on your class and your alignment:
Light Side Jedi Guardian: +3 Strength Dark Side Jedi Guardian: +1 to 8 additional damage Light Side Jedi Sentinel: +3 Constitution Dark Side Jedi Sentinel: +3 Dexterity Light Side Jedi Consular: +3 Wisdom Dark Side Jedi Consular: +50 additional Force Points Jedi Weaponmaster: +3 Strength Jedi Watchman: +3 Constitution Jedi Master: +3 Wisdom Sith Marauder: +1 to 8 additional damage Sith Assassin: +3 Dexterity Sith Lord: +50 additional Force Points
Your status screen only shows one bonus at a time, at the top. However, bonuses from your starting class are still present and active. Two bonuses of the same type will stack. For example, a Light Side Jedi Guardian / Jedi Weaponmaster has +6 Strength (+3 Strength for each class), and a Dark Side Jedi Consular / Sith Lord has +100 Force Points (+50 Force Points for each class). The great way to be a strong Force User is to be a Light Side Jedi Consular / Jedi Master. You will get Master Force Focus to add +4 DC to your Force Power Saving throws, and +6 Wisdom for Light Mastery, which increases your Force Points, Force Damage, Force Power DC and, for male characters with Battle Precognition (learned from Handmaiden), a +3 bonus to defense for Light Mastery.