Politician and and a banker.
Doctor and the FDA
serial killer and cannibal.
It's probably a case of formulating general rules in lieu of deciding 'yup, those two'.
Among the considerations may perhaps be which combos you are able to setup - there are several professions that are effective in placing fields (engineer, elementalist, some necromancer and mesmer setups) and some which are great at utilizing them (http://www.guildwars2gold.org/ ) It's probably easier to have considered one of each in lieu of placing plenty of fields and not to be able to exploit them, or having lots of finishers and zip to complete.
Elementalists specifically often perform a lot better after they have someone else for that mobs to focus on - they're pretty squishy, especially for the lower levels where their selfheal is similar to locating a bandaid over a severed artery.
Nevertheless, which professions the both of you enjoy most will likely make even more of a change.
Doctor and nurse
Lawyer/doctor/insurance saleperson = Vampire