"double cast" seems overpowered. I think a 10min recast like "seals" more appropriate. However, I'd go with "quicken cast". Ideally, use of this job ability would push the level of "fast cast" to a +1 or +2 effect. For example, tier 1 fast cast at whatever level would be considered tier 2 or 3. What's the finally tier IV or V? Either way, they would become tier V-VII give a take a tier depending on how quick this /JA ability worked.
Yes, admittedly subbing scholar would give this same effect but then a redmage would be locked down to that sub job.