Back to Double Cast though, I actually can think of quite a few situational uses for this the way the OP originally presented it - 2 insta cast spells. Not only two quick raises and a way to insta-cast reraise consistently, but you could also use it to Double Cast stoneskin if you're getting beat on and don't want to risk the hate of a couple Cure IVs. Assuming it would also reset the second spell's recast timer to 0, I might even use it to Double Cast gravity on something I'm kiting that could possibly resist it. Similarly, if your Slow II can possibly get resisted (?), Double Cast Slow II for an extra chance of landing it without waiting for recast. Or if you're soloing on /NIN you'd have a bunch of successive shadows here. /BLM or /DRK and three quick stuns every five minutes. And my favorite, Double Cast instant Warp II's, Warp or Teleports just for saving time. Not big things but they're getting me thinking.
Also, I don't consider this already in the game because Quick Magic exists. It's a huge meaningful difference that you could use Double Cast intentionally rather than it firing off randomly.
Having said that, though, I still view this as more of a neat toy than anything super useful. It wouldn't do much to make RDM more attractive to current party set ups, which is what the job needs right now. In that sense I'd still go for Runic first, and second, a Double Cast that allows simultaneous casting to cut down on cycling time, despite its obvious problems with equip swaps. Although even those two wouldn't be enough.