Can one enemy be attacked twice per round with one player using ithilien archer to move him back to staging and then 2nd player attacking with dunhern?

Yes.

A player cannot declare multiple attacks against the same enemy, but different players can.

Obviously they both need to be able to attack to do this, so this needs ranged or some other mechanism, such as the one you described.

Although one Ranged character with readying could attack the same enemy twice.

1 hour ago, Wandalf the Gizzard said:

Although one Ranged character with readying could attack the same enemy twice.

Just to add, this would only work in a multiplayer game as each player gets to declare one attack to a target. The ranged character, if ready, can join each player's attack against the same target.

Being ranged doesn't inherently allow for multiple attacks on the same target.

1 hour ago, TheGuardian118 said:

Just to add, this would only work in a multiplayer game as each player gets to declare one attack to a target. The ranged character, if ready, can join each player's attack against the same target.

Being ranged doesn't inherently allow for multiple attacks on the same target.

Correct. Thanks for the clarification.