Haven't reached a point where I can choose 2nd jobs yet, but here is what I've chosen and why.
Vaan - Knight (will be taking Bushi) Vaan has really good magic stat (3rd to Penelo and Ashe) and his animation speed for greatswords and katanas are top tier. For lore reasons I would have liked to give this to Basch but his low magic stat was a roadblock.
Basch - Shikari (will take Foebreaker) he has the fastest dagger and ninja sword animation in the game (actually he's great with pretty much all weapons) and this combo does not require any magic at all. His low vitality will mean slightly weaker damage with hammers or axes, but he won't really be using them that often.
Balthier - Uhlan - Balthier has top tier speed swinging spears, so he's perfect for this role. I believe I'll be taking time battlemage as his 2nd job, but there's an outside chance of taking machinist- I'll decide later. Both of these jobs will provide "swiftness x3" and both give some access to time magic. Heavily leaning on time battlemage though since I'd rather not give it to my white mage.
Fran - White mage - she doesn't have a great magic stat, but decent enough, and if there's one category of magic you can use with non-optimal magic stat its white mage - you want your offensive spell casters to be your best magic users. 2nd job will be machinist or time mage, depending on what I want for Balthier.
Ashe - Black Mage (Monk) best magic user in the game combined with tons of buffs and utility from monk, including high level white magic (holy)
Penelo - Red Battlemage (Archer) - natural combo here as Fire Bow can power up ardor significantly. Only other option to do this is black mage / red mage so you can use flame staff, but I'd rather separate my offensive mages. With Penelo's high magic stat, she will decimate enemies. Having ranged attacks from bow is nice for some situations, and the swiftness x3 from archer ensures faster cast times.