I demand a more in-depth explanation of how you think such a system would be truly unique, effective, balanced, and whether or not it makes any sense. Although, to expand on this idea, perhaps you could also marry the DoL and DoH to be able to add certain properties and dimensions to battle; FSH/"SCH" for instance, would be a an Advanced Job of many different combinations on account of the Adv. Job being "unlocked."

Then let's say... FSH/SCH is able to have more familiarity with water and wind, or perhaps more damage to Scalekin and things fish-related; or MIN/PLD could have upped defense due to their familiarity with say... Ice and Stone so they're more capable of upping their own defense b y some....means...

Or let's imagine ALC/SCH could be something akin to an Alchemist/Creator from Ragnarök Online if you can imagine throwing magical alchemical potions at things and watching them explode like Acid Demonstrations on a Paladin before "renewal."

Or even ALC/SCH could be a potion-throwing, heal casting monster.

Then again, think of the major cost of production in complexity of such a system alone. It might be a while until a game can successfully develop a game that is able to have a high degree of complexity, a healthy balance between a broad band of class/job combinations, each combination a little unique to everything else combat-wise, practical, and be able to afford the technical know how to get such an accomplishment any chance of happening. Then again, if you consider the six hundred or so employees they've had working on FFXIV:ARR at one point (Which was rumored to include people "supposedly" working on FFXIII Versus), plus consider how much work they've supposedly done to the game while "a lot more" is in store. If you also consider the fact that the Companies Honor is at stake, they're trying to each back their customer's trust, and the likelihood that they're burning the midnight oil on a regular basis.

TL:TR

It's possible due to their huge number of employees and other factors, however unlikely.