If you have Dunhere and any ranged character with the Great Yew Bow, each of them may target and attack the same enemy in the staging area if you wish, but none of these effects will allow you to combine these attacks. But what about Bard the Bowman and his -2 defense? Does it accumulate?
You, as a player, are only allowed to declare one attack against each enemy during the Combat Phase. However, cards like Great Yew Bow, Quick Strike, and Hands Upon the Bow all include a declaration of attack as part of each card's effect. They do not affect or in any way conflict with your normal ability to declare an attack.
great yew bow:
Combat Action: Choose an enemy in the staging area. Exhaust Great Yew Bow and attached hero to make a ranged attack against that enemy. Declare attached hero as the attacker. No other attackers can be declared for this attack.
Action: Exhaust a character you control to immediately declare it as an attacker (and resolve its attack) against any eligible enemy target.
hands upon the bow:
Action: Exhaust a character you control with ranged to immediately declare it as an attacker (and resolve its attack) against an enemy in the staging area. It gets +1 [Attack] during this attack.
Bard the Bowman
When Bard the Bowman makes a ranged attack, the enemy he attacks gets -2 Defense until the end of the phase.
As an example, let's say you've got Dunhere and Bard the Bowman, each with Unexpected Courage (Bard the Bowman also has Great Yew Bow), and "Big Enemy X" is in the staging area. During the Combat Phase, you could play Upon the Bow on and Bard the Bowman and have him attack the "Big Enemy X". Then play Hands Quick Strike on Dunhere to have him attack "Big Enemy X". Ready each Hero with Unexpected Courage, then use Great Yew Bow with Bard the Bowman to attack the "Big Enemy X" a 3rd time. And finally use your normal declaration of attack on the "Big Enemy X" with Dunhere to attack him a 4th time.What defense does "Big Enemy X" have at Dunheres second attack?