Hi,

If I have two Booming Ents in play and one has 1 damage on it, in this case both Ents should have 3 attacks points, right?

Booming Ent gets +1 [Attack] for each damaged Ent character you control.

Thanks,

Correct.

Using my dark necromancy to bring this thread back to life... So is it everyone's understanding that damage on Booming Ent makes himself count as a damaged "Ent character", and thus grants himself the +1 [Attack]?

Yes that is correct