First off, this is not about dealing damage - I have read the FAQ and know that if there is any damage to be dealt when using multiple defenders out then all of said damage goes to one defending character.
Instead, it deals with how to apply the card to attacks. Here's the text from the card (my emphasis)
"Action: Chose a player. That player may declare any number of his eligible characters as defenders against each enemy attacking him this phase."
My question is: how does this play into the four phases for "Resolving Attacks" as laid out on p.18 of the rules? The wording of "each" and "this phase" seems to me to imply that the chosen defenders are used to defend against all the attacks that turn. In other words, for each enemy engaged with the player that phase that is picked in step 1, the defenders declared with Stand Together are added to any other defenders picked in step 2.
For instance, suppose I am engaged with two King Spiders (3 damage each) and Ungoliant's spawn (5 damage). I have three Veteran Axehand in play (1 defense each). If I play Stand Together and select the three Veteran Axehand allies, then using my interpretation and declaring no other defenders, if I resolve the attacks from the King Spider's first, the grouped Axehands block all damage from each King Spider (3 attack - (3x1) defense = 0) and then Ungoliant's spawn deals two damage (5 attack - (3x1) defense = 2) to one of the Axehands.
Previously I had read this card and thought it wasn't worth the trouble since I could only pool defenders for just one attack -- but if it's the case that they can be pooled for all the attacks, then this becomes a much more powerful card - especially when the Trolls attack at the Carrock!
Of course, it's possible that the intended interpretation is just "you can defend each attach with more than one character this turn" which, while better than what I thought, is not as good as what I outlined above. I guess it comes down to a scope issue: are the characters who are standing together resisting all the attacks, or is just that each attack may face separate groups of characters that are standing together against just that attack?