Both the equipped monster and the Union Monster can attack after “Combination Attack” is used, assuming the Union Monster was Special Summoned in face-up Attack Position.
“Combination Attack” can only be used against a monster that reached the Damage Step; if a monster attacks and a replay occurs, then it is not considered to have attacked for purposes of “Combination Attack”.
You can use this effect on an equipped monster on your opponent’s side of the field that has attacked. Your opponent’s monster can then attack again as per “Combination Attack”’s effect.
You can attack with a Union Monster, use “Formation Union” to equip it to a monster, attack with the equipped monster, activate “Combination Attack” to detach the Union Monster, and attack again with the previously-equipped monster and with the Union Monster.
If “Dark Blade”, equipped with “Kiryu” and “Twin Swords of Flashing Light - Tryce”, attacks twice using “Twin Swords of Flashing Light - Tryce”’s effect, then you use “Combination Attack” to detach “Kiryu” from “Dark Blade”, both monsters can attack again once each. “Dark Blade” cannot attack another 2 times because it has already attacked twice.
If the player who would control the Union Monster already has 5 monsters on the field, “Combination Attack” cannot be activated.