The normal two-under approach has the person not bidding the two-under call bidding the lower suit if he prefers that but the one-up artificially to show a preference for the higher suit. Thus, if 2C shows one or both majors, partner bids 2H with heart preference or 2S with spade preference.
If the stronger hand bids the two-under, you could reverse the meanings, indicating your least preferred suit. Thus, if 2C was bid by a strong hand to show one or both majors, Responder/Advancer would bid 2H if he prefers spades but 2D if he prefers hearts. As a pure coincidence, this happens to mean that the weak hand transfers to the suit he prefers. If the strong hand has both suits, he accepts the transfer. With only one suit, he bids that suit. Using this "transfer to the suit you prefer" method, the strong hand always plays the hand when the strong hand is two-suited, and the strong hand usually plays the hand even when he is one-suited, as the transfer is to the least-preferred suit (which is more often what the strong hand actually has).
It might be rare for a strong hand to use the two-under approach, but it could happen. For example, one might devise a scheme where a 3S rebid after a 2C opening is forcing beyond 3NT and shows one or both majors, and huge. 2C-2D, 3S-? In that scenario, as an example, Responder could transfer to the suit he prefers, and the two-under scheme still works great, just with the two-under bidder more often declaring.
Your reaction might be to suggest that transferring to the suit you prefer is easier to remember and maybe should be used all the time. But, the most common use for a two-under (one or both of two suits) approach is in situations where the person bidding two-under is weak, and where therefore you generally want the lead into the stronger hand (and the less described hand hidden).