You can't stack them, but you can use it back-to-back-to-back. I should use mine to finish up my subs. My BLM is at 45 right now (and I'll probably leave it there for a while since I don't imagine we'll go past level 90 until December's final update anyway). I'm not entirely convinced that I'll need WHM past level 80 (I imagine /SCH will eclipse it), but I should level it past 41 regardless. NIN is my primary concern, but I think I'm just going to try and get in an astral burn for that anyway.

This would really, really help me finish up my merits before the level cap, though. I need four more merit points to finish up RDM group II merits (don't really care about the additional magic accuracy from group I, and for a taru convert recast timer is a joke), and I still need 20 more points to finish up SCH merits. Then there's elemental magic, MP, and some other assorted things... ugh!