I can only see this happening if the Pokémon Solgaleo and Lunaala "served" was one and the same, regardless of which game you had. Ignoring gen 1 and overlooking gen 2 (which had both Ho-oh and Lugia obtainable, just at different times), you could argue that one of the main reasons why they place the "mascot" legendary on the cover is because that's the "main" version difference between games: if I get Moon, it will be tough to get a Solgaleo because it's a legendary that only exists once in each game of Sun--unlike other version-exclusives which can be found multiple times in the wild or bred to then trade for the one(s) you're missing. So if, say, Lunaala worked for Mr. Moon and Solgaleo worked for Ms. Sun, we'd kind of need them to be on the covers because the Average Joe who is perusing the video game section of the local store will want to know what major exclusive Pokémon to look forward to when making his decision on which game to get. If Solgaleo and Lunaala both served Sir Star then that's a little more okay because it's not exclusive to either game. It really comes down to good marketing tactics: they want to feature the "major" legendary on the box to catch peoples' attention. So if they were to do this kind of "switcheroo," it would almost have to be a singular Pokémon that both Solgaleo and Lunaala serve--kind of similar to how the "third major legendary" is obtainable in both games of the "main pair" and then all three are obtainable in the third version (Ruby/Sapphire had Rayquaza; Diamond/Pearl had Giratina), except with more of an impact on the plot.