I can attack an enemy in the staging area with Bard and Great Yew Bow,then attack the same enemy with Dunhere(taking into account Bard's -2 Defense)?

No, there is a limit on attacking. You can only attack each enemy once (unless of course you find an event card that enables you to do otherwise), it's in the FAQ(1.11).

Quick strike and hands upon the bow are currently the only events I know of that let you do this.

Yes,but declaring one attack with Great Yew Bow is different from a normal attack. I think it was a confirmation somewhere on forum.

I can attack an enemy in the staging area with Bard and Great Yew Bow,then attack the same enemy with Dunhere(taking into account Bard's -2 Defense)?

It looks like a bonus attack to me which would be similar to the rulings regarding Hands Upon the Bow or Quick Strike (You can use them and still attack normally). However, the wording on Great Yew Bow is slightly different than on HUtB or Quick Strike so perhaps it doesn't count as a bonus attack.

It seems legal to me, unless the developers have ruled otherwise.

No, there is a limit on attacking. You can only attack each enemy once (unless of course you find an event card that enables you to do otherwise), it's in the FAQ(1.11).

Quick strike and hands upon the bow are currently the only events I know of that let you do this.

The FAQ just says that it has to be a "card effect" not an event. Great Yew Bow is a card effect, so I think it should work.