Even though combining Chainspell with Stun might come back in vogue once Embrava and Perfect Defense are adjusted, there's the counter-argument that two Scholars would have low enough Stun recasts for the entire event rather than a 120 (140) second burst of TONS'O'STUNS.
CSS is good and dead for 90% of things you need them on. RDM only has an E in dark magic skill and due to the way skill caps raised after level 76 RDM's dark magic is so low that even stun's magic acc bonus isn't enough. SCH's on the other hand have B+ in dark magic with DA up, that is before their gear and staff options.

RDM @99 = 300 base Dark Magic Skill
SCH @99 = 404 base Dark Magic Skill

A level 110 monster had 450 base magic evasion, and gains another 7 for each level. This doesn't take into account any natural resistance to thunder nor any resistance (natural or accumulative) to the stun status effect.

So yeah two x SCH/BLM's can stun lock pretty much anything.