My thoughts were more from a Tank standpoint thus why MRD or Gladiator as the primary and Thaumaturge/Black Mage as the secondary as for lore purposes... their power comes from the void. Which is much stated to be a negative place. So it makes some since to me in that matter and the second cross class would be dependent on what the primary would be.

As well i put Gladiator on the spot because whenever they are presented along side Paladins they are always two halves to the same coin. Using usually the type of weapons and armor.

I wholeheartedly say it would be Gladiator 30/Thaumaturge 15 to unlock Dark Knight.

Also i dont know the full mechanics and playstyles for the two tank classes so my information is just based on what little i currently know as well as what i know to be established both in 14 lore and in the lore of paladins/dark knights.

It is possible that they take a tank base and go DPS much akin to the Arcanist becoming the healing Scholar.