Official Ruling
No, you can't. You can activate the effect itself twice, but after the attack declaration is made and effects such as [When Attacking] effects have activated for your opponent's 1st Digimon, the 2nd card's effect activation timing will occur before the counter timing. However, since the 1st Digimon is already attacking, its attack will continue and an attack declaration can't be made for the 2nd card.
Source: EX3-024 Slayerdramon