Steady breathe or warding breathe

Making them focused tanks make them more effective on their focused stat but Mixed tanks do well on both but do'nt excel either.

IMO make them mixed tanks. cause it will be More generalized and is more easy to play
Though you can Focus tank when it is needed.

If they're playing on a team together, I suggest Fae focus slightly more on res while Nowi focus slightly more on def. The whole thing is just more efficient this way because Fae is better at res to begin with, and the same is true of Nowi with def. So for example:

Fae: Lightning breath +def (because 29 is a bit too thin for my liking), warding breath, distant def
Nowi: lightning breath +res (to deal with red mages), steady breath, close def.

Close def is especially good on Nowi now that the Tikis got anti-dragon prfs. Nowi is the one dragon they consistently have problems taking down. They can't survive two hits if Nowi initiates (but yours won't double if she initiates); they might not double if they initiate (Y.Tiki will probably double your Nowi though), and it's quite hard for them to OHKO due to colour disadvantage.

Nowi will not be comfortable vs strong bows and daggers (only 37 def before buffs, and by strong I mean atk in mid to upper 50s), but she will have enough. You will still need someone to take care of axes because Fae getting doubled with 33 def might not work. However, she has green and blue mages covered with 40 res.