An auction that I have seen rather frequently as calling for a lead-direction indicator is on that I describe in my "Really Unusual Notrump (R.U.N.T.)" book. An example:
In RUNT, I recommend 2C as agreeing diamond to declare but preferring a spade lead, 2H as to declare spades but wanting a diamond lead, 2D or 2S netural but leaning to lead that suit. That helps when Advancer has, for example, Kx or Ax in one suit but 3-4 small in the other.
This works wonders if the opponents end up playing hearts, where the person on lead has the information now as to which suit to lead. This then made me think whether this same type of messaging might be used to enable a person showing two suits to indicate his lead preference.
Suppose, for example, this auction:
Sandwich is less important as a range describing tool, as we know that Dealer has a weak hand. One could, of course, define X and NT as different packages of suits. For instance, 1NT might show the minors notwithstanding the 1C opening. But, perhaps both partners can discuss lead preference, to cater to a final contract of either clubs or hearts as the strain, and hence either person on lead. (Maybe there are other circumstances where this discussion makes sense; the purpose here is to discuss theory and tools, not judgment.)
Suppose, then, that the 1NT call showed lead preference in the lower suit, X lead preference in the higher suit. You might then have these possible auctions:
Overcaller prefers a diamond lead against a club contract; Advancer wants a spade lead against a heart contract but agrees diamonds as our fit. Had Advancer bid 2D instead, he is neutral or prefers diamonds for lead also.
Overcaller prefers a diamond lead against a club contract; Advancer prefers a diamond lead against a heart contract but wants to declare spades. Had Advancer bid 2S instead, he prefers a spade lead against a heart contract or is neutral.
Doubler prefers a spade lead against a club contract; Advancer preferes a spade lead as well but wants to declare diamonds; had he bid 2D he would be neutral as to lead or prefer a diamond lead.
You get the point.
What inspired all of this was a sick hand from last night. The opponents bid 1C-P-1D, to me. With AQxx in spades and Jxxxx in hearts, I really wanted a spade lead. The end result of the auction was that my RHO ended up declaring 3NT, where I was on lead. My lead was a heart to partner's Ace, partner got in, and a spade return would hold the opponents to 10 tricks; they ended up with 11 when partner continued hearts. Our defense should have resulted in three tricks for the defense, but that is not the point. It would have been fantastic to have the auction develop where I knew to lead the heart and did not just guess, and where partner knew to switch to the spade without any guessing as well. This could have happened by me offering both majors by way of a call that indicated preference for a spade lead and partner picking hearts with preference for a heart lead (or neutral).
Granted, this scheme would cause potential problems when acting as overcaller, because you might have equal holdings in the two suits. But, the solution there is to have one of the two calls show preference for a specific suit or neutral. One would be clear.