I'll compile a list of why and why there shouldn't be dual advanced classes based on feedback and my own opinions, as well as solutions to allowing them. I'll start off by listing why I think there should be both advanced classes available(at a certain point).
Pros/Why
-Feeling more bonded to your character
-Less time spent leveling alts(depends on whether or not you enjoy it)
-Less time spent gearing alts(datacrons especially)
-More trade due to people(who don't enjoy leveling alts) buying craftable items for both of their ACs
Cons/Why Not
-People may need for other AC/Loot-whoring/Spec-swappers(Solution: Master looter or need only available if the armor type is equippable for that AC; Due to no cross-realm LFG, you don't have to group with a person again[ignore them])
-Less diversity(Switching between two characters isn't any more diverse than using the same character)
-Possibly less materials/crafted items for certain professions due to limit on a single character
-ACs are completely different classes(True to an extent, but each AC shares the same primary stats so ACs are really just different playstyles)
-Breaks the game(No reason needed, apparently)
Solutions
-At level 50, make a quest chain for each class(large preferably) that allows you to respec one or both of your specs to either AC aka you can have one spec as a sage and one as a shadow, both shadows and the ability to change one to sage, etc. and only have to pay normal respec price to to switch a spec between either AC.
-"Leave as is. ;)"
This is not what BioWare says, it's pros and cons, and your opinions/thoughts on them.