BLM duo partnerFollow

Back before Abyssea, the answer would have been to have you both go BLM and nuke beastmen pets for exp. Abyssea took over as the prime exp method for higher levels. Currently, Grounds of Valor is the prime exp method for lower levels. For GoV, you want to start out doing GoV pages with just the two of you and look for other people to join up with on the spot. Groups don't form by /shouting in Jeuno for GoV. Players just show up to the spot, start doing their thing, and band together as they go. Since the bulk of exp from GoV comes from page completion, a reward that doesn't get decreased for larger groups, it doesn't hurt anyone's exp to grow up to a full alliance. In fact, being in a full alliance is better exp, since no one has to compete for respawns.

Getting exp has become really easy. It's so easy that you really don't need to go out of your way picking a job to duo with your wife's BLM (although WHM probably would be the hardest).

Instead, I suggest you pick a job based on what would make your duo more effective for end game activities. THF + BLM would be an excellent choice for Abyssea farming for seals. NIN + BLM or MNK + BLM would also work quite well.

These days, it's far more common for players to have multiple jobs leveled than it was in the past. With that in mind, you might even want to consider BLU + BLM. Once you have BLU leveled and geared appropriately, you can AoE burn groups of mobs in Abyssea just by yourself so that your wife can level a new job. My friend who dual boxes specifically leveled BLU on his 2nd char to make it easier to level new jobs on his main char as well as farm Key Items in Abyssea.

I'd have to give a rather significant advantage to BLU for AoEing in Abyssea what with whisker being a one hit kill.