Say you use Tactics' Stand Together to declare 2 defenders against one attacking enemy. Do you add the 2 defenders' defense strength and subtract the total from the enemy's attack strength and remainder, if any, is damage dealt to a single defender (as mentioned by offical FAQ). I don't find anywhere that you total all the defenders defense strength when you use Stand Together. But what use is ST then?

Yes you add the Defense Values, otherwise would this card make no sense at all.

I think most would agree that if you don't add up the defenses, ST is useless. But most (all?) also play it that you do add the defenses together. I think ST just changes the attack minus defense = dmg by allowing multiple defenders, so when you deduct defense, you deduct defense from all defenders. Any leftover still has to go on one character.