You could dual main pretty much any two of the top tiers and get some benefit out of it. Even if they share a disadvantageous MU (or one that you're uncomfortable with), you'll be getting more insight into how the MU should be played between the two characters, and have two ways of approaching it based on what skills you presently feel on point with.
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^all combinations that I've seen at least somewhat frequently. Players who main pretty much any char below that try to have one of the top 4 as one of their mains. A lot of these choices have to do more with comfort/similarities in movement, spacing, options, character strengths, etc. For example...Sheik+Falcos aren't very common. Probably because they differ in every one of those factors. That doesn't mean less common ones don't work though lol. Zhu manages, and one of the better players in my region uses the pair too.
So if you're lost and trying to pick between a few characters...keep the 1-2 character(s) that feel most comfortable to you right now. If neither are in the top 4, pick up whichever top tier feels most comfortable movement-wise, or - even if they aren't - whichever plays similarly to your other mains (spacing/options/etc.), since you can just adjust to the movement with time. If none seem like a good fit, keep playing the top 4 chars as pockets for friendlies and practicing tech. Over time, you'll be sure to start finding one more comfortable than the others, and maybe have it outgrow your lower tiers (which you should still keep, if they can keep up; competent mid+ tiers can help if you're getting outplayed in common top tier MUs). You'll reach a point where even if you have 1-3 decisive mains, you can and should be able to play most high/top tiers at least somewhat competently, if only to explore and be more aware of their options.
TL;DR: The disadvantageous MUs of the top 4 characters balance out a bit more as you get better. Since the whole point of dual maining is that you're able to play any MU just as competently with either character (excepting the worst of your MUs, for below-top-4 characters), combinations shouldn't be heavily influenced by MU spreads. What's more important is being able to play them both competently/comfortably, and being able to rely on a second option if you're getting outplayed or flubbing up with the other.