Second JA to second mnk: +35, for a total of 228.
Second JA to sch: 16.25 + 16.25 + 12 = +44.5, total of 237.5.
So solely comparing who you give the JA cell to (assuming the sch already has magic) ends up with more total damage by giving the sch JAs.
However you also argue the utility of giving the magic to the rdm instead for Haste. Haste would improve the three melees' damage by about 17.5% (about another +38 units (doesn't affect counters)), though at a cost of 120 MP every three minutes instead of the 24 MP of enspell-ga. Total when giving the JA to the second mnk would then be 266, 12% ahead of the sch option.
Edit/Caveat: That ignores the fairly high odds of getting a magic cell for the rdm within a fairly short time period, which nullifies the haste difference. In the particular parse I'm referencing, we had an overall 60% drop rate on magic cells from the lamia that could drop them. If you get two magic cells in the chest (the scenario that most favors the rdm), you're most likely going to have a third cell within 5 mobs, completely swinging things back in favor of gearing up the sch.