Yeah I'm gonna have to agree, a well balanced game and something like this seems mutually exclusive. I'm gonna compare and contrast this with two other SE titles:
In which case we got a MMO that left the balancing up to players a la Pokemon and a single player game that was balanced by being unbalanced, both were the result of the freedom to use 2 jobs at once, something more extreme than what the OP suggested but still proves that SE would have to be very careful in the unlikely scenario of them attempting something like this in XIV.
- Final Fantasy XI: The precedent darling that everyone loves to compare to. The reason the game was so successful was (apart form not being WoW) the game offered a unique system (in part to the devs being total newbs when it came to MMO design) that allowed you to mix and match any 2 classes. However the balancing was so horrid that it lead to things being soloable in a very group oriented game and single classes like the infamous Ninja being misused. At also lead to things like NIN/RDM and other class comboes being preferred and anyone who deviated from those becoming shunned and derided as scrubs
- Bravely Default: This game also seems to have XI's job system (though it's likely that both cases were inspired from V), however it manages to balanced by making most of the jobs broken (Ninja has Utsusemi, Spiritmaster had Stillness, performer can build up BP allowing extra turns, Time Mage can do the same, Salve Maker can instill elemental weaknesses and exploit them as well get more potency with healing items and Conjurer can give massive buffs). That and the fact that its a Single-player game rather then an MMO means that there more options. However most of the jobs can still rip the game in two. BD has it's own take of crossclassing by using passive abilities from each job which is unfettered so a charater can have a Red Mage/Vampire with Hasten World or a White Mage/Black Mage with Katana Lore