each aren't used equally nor should they actually be grouped together - while you CAN switch back and forth most players who play sheik do it because they enjoy sheik and most who use zelda do it because they enjoy zelda. the playstyles aren't even remotely similar - so it's not entirely likely to see an equal use sort of player. sheik's use zelda for the kill or zeldas use sheik for certain matchups - but there is always a central focus on one of the characters.
bleh
I've gone to tournaments and played Sheik and Zelda because i enjoy Sheik and i enjoy Zelda. I used the two equally, with little regard to the fact that their playstyles are night and day. I had no central focus whatsoever, during matches with G&W i switched to Zelda just so he'd have to fight something different from what he fought the first two stocks. You could get picky and say that well i used Sheik two stocks and Zelda one, but if you take that logic without a grain of salt, i could argue that there are only Grenade Snakes and C4 Snakes, some who use grenades to trick people into the C4 and some who use C4 to trick people into the grenades, and we can tally up the number of grenades and C4 after the match to see what character we list the win for.
Say a composer composes a song, and a violinist decides to hold concerts playing it. When the audience comes to see the show they get programs. The programs don't have the composers name alone, nor do they have only the violinist's name. Since each person played an irreplacable role in bringing the performance to the audience, both are credited. When I use Sheik to rack and Zelda for kill, they are each performing essential roles. To drop credit from either character is to imply that it could've been done without that character. But that's not necessarily true, I've played several matches i certainly would've lost without both characters in one match.