It's a bit misleading since many of them primarily focus on one character. MkLeo mostly plays Byleth, Sparg0 mostly Cloud, acola mostly Steve, Kola mostly Roy these days (I think), Maister mostly G&W, Glutonny mostly Wario, Kameme mostly Sora, even Tweek seems to generally default to Diddy Kong these days (although based on his recent YouTube videos he might start using his Sephiroth a bit more). I haven't seen enough of Miya to say whether he's mostly G&W or not.
That leaves Shuton, Tea, Riddles, Dabuz, and ProtoBanham as the players who actually frequently use two characters in top 20. Tea is primarily Pac-Man but his Kazuya is scary, Riddles is primarily Kazuya but he still uses Terry quite a bit, and Shuton/Dabuz/ProtoBanham should probably be considered dualmains at this point.
This list seems to be really generous with considering characters secondaries. Like, Tweek Aegis. It's also worth keeping in mind that many of those characters are like, "This used to be my main and then I picked up another character, but I still kept using my old main to some extent". Shuton was an Olimar main before he picked up Aegis, ProtoBanham was a Lucina main before he picked up Min Min, Dabuz mained Rosalina in Smash 4 and Olimar in early Ultimate and now mains both, and so on and so forth. They're also playing at a higher level than mid level or even high level players, so the opportunity cost for playing two characters isn't as significant.
I think if you actually want to pick up a secondary, you should probably treat it as a main for at least a few months. If you just play the character occasionally but never really dedicate yourself to it then there's a good chance you're going to do worse than you would've done with your actual main.
MkLeo
acola
Light
Sonix
Riddles
Glutonny
