You at least know the shape of the katakana "ni" and can guess it might be "ni" and not the kanji "2", can't you?
(Just like you once came across 健一 vs 健ー.)
So, why don't you check both カンニング and カン二ング in dictionaries, as already pointed out? Then you will be able to get the correct answer by yourself without asking us.
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