While resolving Samut's last ability, all of the creatures and planeswalkers put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they'll see each other.

If Samut's second ability has two targets and one becomes an illegal target, the remaining target is dealt 1 damage, not 2.

You divide the damage as you activate Samut's second ability, not as it resolves. Each target must be assigned at least 1 damage. In other words, as you activate the ability, you choose whether to have it deal 2 damage to a single target, or deal 1 damage to each of two targets.