I just came back to the game a few weeks ago after playing at release and off and on since. I probably am not the best to answer the question, but I will say that you will want to level multiple things into at least the high teens anyway.
FFXIV works differently than FFXI in that you can use other classes abilities as long as you have leveled high enough on that other class to get the ability in the first place.
This means, that if one of you wants to play a caster (DoM) style character, you will want to level both THM and CON some to get the good stuff from each. This will also help you decide what you want to play the most.
Personally, though I am a bit lower level still, if I was going to duo with someone a lot, I would want to go PUG or LNC with a few tank moves for good measure, and have partner as a THM main with some CON buffs and cures as well. ARC can be good solo too, I hear, but I don't care for it personally.