The two-under approach has some good apps when the two suits are both minors (and the artificial call is made in spades, with 2/3NT for diamond preference) or both majors (and the artificial call is clubs, with 2/3D for spade preference).
What about the red suits? This would be of more limited application, as it is not easy to think of circumstances where you would want to dedicate a call to showing both reds or just one red suit and where ypou would want to give up another meaning for the call -- notrump -- that is two under.
An example of how it would work, whether useful or not, might be to ovbercall 2NT after the opponents bid and raise spades as a red(s) call:
Overcaller would have diamonds, hearts, or both red suits, with Advancer bidding 3C if he prefers hearts to diamonds. That might allow 3D or 3H directly to show extras.
Another similar example might be in a more competitive auction. Suppose that you open 1D and the opponents compete in spades:
This might be the way to show:
(1) diamonds, competitive
(2) four hearts, competitive
(3) THREE hearts, competitive
Responder would then "prefer hearts" if he doubled with five hearts. That gives up showing clubs naturally for Responder, or Opener using 2NT possibly with clubs, such that it might not make sense to use this method. My purpose, however, is more to discuss how it might work if you did, simply to think through the methodology.
The remaining, touching two-suiter is clubs and spades. But, they do not really touch, as notrump is between them. But, one could imagine nonetheless using diamond calls to show both and hence a heart response as club preference:
1NT-2D as spades plus clubs, or just one of them
1NT-2D-P-2H as club preference
Again, the usefulness of this tool seems dubious, but the mechanics is what interests me. I find this interesting to think about because I did not think of the clubhs-for-major(s) parallel to the spades-for-minor(s) despite playing the latter for years now. Maybe diamonds-for-black(s) or notrump-for-red(s) might fit in somewhere I have not yet recognized as a curious solution.
UPDATE: After thinking this over with my morning coffee, I thought of something where a lot of this could be used; after a 1NT opening, if you wanted to be very precise:
X = red(s). Advancer bids 2C if he prefers clubs. Doubler then places the contract. This handles reds, hearts, and diamonds.
2D = blacks. Advancer bids 2H if he prefers clubs to spades. Doubler then places the contract. This handles blacks, clubs, and spades.
So far, all one-suited hands are covered, as well as two of the two-suited hands.
2C = majors, or either major and the non-matching color minor. Advancer picks his major of preference, bidding 2D to preference spades. This is a limited Crunched Capp, keeping the major-minor to a specific major-minor.
So far now, all normal two-suited holdings are covered, except both minors.
2M = canape with shorter major, longer minor
2NT = minors
This structure (again, only allowed if not GCC) allows the partnership to show any one-suiter (clubs at the three-level, which is a common problem), allows specific identification of the minor when major-minor, and allows showing canape major hands.
To make this fully GC legal, have 3C direct to show just clubs; 2D would then show spades, or spades plus clubs, such that 2D guarantees spades.
Again, this might not be ideal, and perhaps even more could be stacked on, but you may see the potential now and why I am thinking about it.