Since GoD, you can only opt for subclasses on an awakened character (so automatically 85+). Keep that in mind; the level 75 requirement is only for adding 2nd and 3rd subclasses.
Ghost Sentinel is a quite nice class, but still it is much more PvP oriented than PvE (doesn't matter open field or instances), so for a DUAL class, you would want to play a support (healer/tank/iss) or a PvE class (titan, em, gk, etc.)
For the remaining 2 subclasses, just pick whatever you can level to 80 fast. Assuming you want to reuse A/S grade Paulina's gear that would be Moonlight Sentinel and Saggitarius (simply because farming mobs with a ranged class is sooo much easier).
There is literally NO USE of these two subclasses once you get to 80 and receive certification. Once you are super stacked you may want to reset one of these to level 40 titan and go farm QA. But that's from a purely theoretical PoV.
Ideally I would choose Dual Class, Subclasses that can use same Armor(anything except Wizard or Tank)... you get new untradeable Paulina's Gear for each Subclass so if you pick classes that can use Light Armor it will keep your WH from flooding(Destroy what you don't need) https://l2wiki.com/Paulina%E2%80%99s_Light_Armor_Box_(S-grade)
When you Awaken Main you'll get 30-Day R Grade(and Diplomas, just get SSR)... so don't destroy the S-Grade, you may need it when the 30-days are up.
Since GoD, you can only opt for subclasses on an awakened character (so automatically 85+). Keep that in mind; the level 75 requirement is only for adding 2nd and 3rd subclasses.
Ghost Sentinel is a quite nice class, but still it is much more PvP oriented than PvE (doesn't matter open field or instances), so for a DUAL class, you would want to play a support (healer/tank/iss) or a PvE class (titan, em, gk, etc.)
So If I want to go healer for a DUAL class, I have to get a robe armor set right?
So If I want to go healer for a DUAL class, I have to get a robe armor set right?
Awakened Healers do ok with Light Armor... they get Superior Armor Mastery Passive, which is both Light Armor and Robes combined.(Robes will give you Higher Max MP) https://l2wiki.com/Superior_Armor_Mastery
While YUL/OTHELL/ISS/TYRR/WYNN each get slightly different versions of Superior Light Armor Mastery(WYNN also gets robe, TYRR also Gets Heavy and ISS gets all 3 types) https://l2wiki.com/Superior_Light_Armor_Mastery