Giving each job a second spec: would it work?

[Delily]

As for the numerous jobs they could add, I don't think it's really a good idea. The more jobs you have, the less you can keep the game's balance. FFXI is a pretty good example of how lots of jobs are just useless, even it they tried to rotate them with each patch.

FFXI's problem is that the jobs did not have well defined roles. Ninjas are considered tanks, but they were originally intended to be damage. Summoners are a mixed bag. The trinity system may seem constrictive, but it sets limits on the expectations and abilities of the job and gives the designers the freedom within those limits.

To answer the OP's question, no it wouldn't work. One spec would always perform better than the other, and the community would expect players to use that spec for content. As many have pointed out, FFXV version of skill trees and specs is the ability to switch your job at any time (outside of instanced content).