Every job or class in the game scales off of a singular stat for its output. RDM shares gear with BLM and SMN, so everything it does scales off of INT.
Vercure is kind of a special case that's shared with Clemency (a PLD spell) in that it's a healing spell that doesn't scale off of MND - it scales off of your primary stat instead. It's a little unintuitive since MND is otherwise established as the healing magic stat, but it's done this way because gear design is done in a way so as to make this the only real option if healing magic as a utility is to be brought to something without substantial MND in the first place.
As far as Dual Casting, it operates on a very specific principle: Whenever you cast a spell that uses a casting bar, Dual Casting triggers, removing the casting bar from your next spell that would have one.
This means that Dual Casting will not trigger on a spell that has had its casting bar removed by Dual Casting, or a similar effect such as the role action Swiftcast.
This means that you would want to make the most out of it by making sure that you never utilize the casting bar on spells that take a long time to cast - you can cast a spell with a short casting time in order to remove the casting bar from a spell that would normally have a long casting time.
Your spells are basically divided up into two categories with regards to this: Fast spells and slow spells. Look over your spells - if it has a cast time of 2.5s or lower, it's a "fast" spell, if it doesn't, it's a "slow" spell.
So basically, you save time and are able to cast more spells by doing normal casting of your "fast" spells, and using Dual Casting to instantly cast your "slow" spells instead of using their long casting times.
Conveniently, all spells that come up via a proc (e.g. Verstone and Verfire) are in the "fast" category, alongside Jolt. So, if you ever don't have Dual Cast, cast one of these spells; and if you do have Dual Cast, cast either Verthunder or Veraero (your slow spells). Which one you cast should be in accordance with whichever type of mana you have the lowest on, as you should try to keep them relatively close in value - if one gets too high above the other, you hamper your ability to generate the lower kind until they become more balanced. At 80 of both types, go do your melee combo on enemies, and then go back to casting.
The wording can be a bit haphazard for dual casting, but it's really quite simple; finishing a spell with a casting bar, removes the casting bar for your next one. Once it clicks, the job is actually pretty straight-forward.
