The text of both heroes are:

Brand : Response: After Brand son of Bain attacks and defeats an enemy engaged with another player, choose and ready one of that player's characters.

Merry : Response: After Merry participates in an attack that destroys an enemy, ready another character that participated in that attack"

If one player controls Merry and other Brand, they can declare infinite attacks, because they can always ready themselves.

PD: with this event: The Hammer Stroke , you can kill all enemies in play.

PD2: and with this Thicket of Spears , you are god in the Middle-earth

This is ok? Or is a bug?

It's a combo that has been noticed by folks in the community, and potentially a broken combo that could trigger some errata later on... but...

I haven't heard any 'battle reports' of players clearing the entire table just yet. If Merry has 3 attack and Brand has 3, that is 6 damage without allies or attachments, strong enough to clear any small enemy from the table but not powerful enough to mow down more beefy enemies. And the combo only works if you defeat the enemy being attacked. And they need to be controlled by different players.

Still, 6 attack is enough to clear a whole lot of enemies, including many of the enemies in some of the battle-themed quests, so I wouldn't be surprised if we see a future errata put a limit onto one or both of these guys.