If there’s both a level 4 or lower blue Digimon card and a level 4 or lower green Digimon card in this Digimon’s digivolution cards, does this card’s [When Digivolving] effect require me to play both of them?
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Official Ruling
No. The effect reads, “you may,” so you can choose whether to play them. However, if you choose to play them, you must play as many cards as possible.If there are both a level 4 or lower blue Digimon card and a level 4 or lower green Digimon card in the digivolution cards, you can't choose to only play one of them.
Source: ST9-06 Imperialdramon: Dragon Mode