Q: If this card is a Vanguard and I activate the second skill, can this unit "Drive Check" when it attacks?
A: Yes, you "Drive Check" each time this unit attacks if that is the case. This unit only loses "Twin Drive!!", it's not stating that you cannot "Drive Check". This unit also loses "Twin Drive!!" once you activated the skill.
Q: Can I activate the second skill after I attack?
A: No, activation skills need to be activated during the Main Phase, before the Battle Phase.
Q. When does Dragonic Overlord lose "Twin Drive!!"?
A. If you activate Dragonic Overlord's effect during the Main Phase, he will lose "Twin Drive!!" immediately. This means that the FIRST and ANY subsequent attacks, after the activation of his effect, will only allow you to Drive Check one card.
The Dragon who stands above all dragons, bearing the mantle of "Overlord", he is a commander of the Aerial Assault Unit, Kagero, of the militaristic nation of the Dragon Empire. Due to his ability to overwhelm all others in battle, he has earned the title of "The Crimson Reaper" and "The Wind of the Apocalypse". When he breathes his Eternal Flame, a breath weapon comparable to the eternal fires of damnation, all before him tremble in fear at the unending flames of despair.