While I understand the ask and I think it would be cool, classes with multiple job stones have shown to be incredibly difficult to balance. That's why summoner and scholar have only grown further and further apart with each expansion.

Sharing skills, stats, gear that could be used by a base class with two jobs have different roles proved to be unviable by endgame without leaving at least one of them at a handicap. Either scholars skills are too strong for a healer or summoners are too weak for a dps or some combination there of.

The other option of having 2 jobs with the same role, will run into issues where one job eeks out a bit more dps, even slightly. Assuming they share the same general gear but maybe needs different materia melds, would force people to specialize. This would force a lot of people to play the "good job". This is basically the problem WoW has where several specs are considered not part of the meta and are frowned upon in endgame activities. Nevermind you'd be using half the same skills to do the exact same thing, if they're too similar there's no point, but if they're too different it might need to be its own class.

Tl;DR They tried it. It didn't work. Doubt this will ever be revisited. The devs said no