There is no "If", Ukrainian is a language, just because they share similarities (No surprise, they have the same origin), doesn't mean they are the same, nor that russian is the most dominant language in Ukraine. Going by your logic, if only 2/3 understand both russian and ukrainian, ukrainian is still the dominant language, after all, the whole population understands it, unlike russian.
A familiar example: you could say the same for portuguese and spanish, many people in Brazil understands spanish, I'd say around 2/3, that doesn't make us spanish or makes it the dominant language in Brazil, see what I mean? We can understand, doesn't mean we use it