My only hope for the second SP is that it helps a Rune Fencer's whole alliance somehow. The worst magic damage and enfeebling effects in FFXI, like the worst anything and everything, are 10'-20' AoE.

As for what's likely...

Since Elemental Sforzo mirrors Paladin's Invincible, Rune Fencer's second SP might mirror Paladin's shield whacking attack and accuracy lowering SP thingy that was on the test server for a while. That is to say: "Deals magical damage and severely lowers the enemy's magic attack and magic accuracy."