Soul Eater is a good read (ignore that anime ending, as it was made up before the manga had been finished) but the mechanic is more brought up when discussing power-ups. You need compatible soul wavelengths to amp up your strength by making them "resonate" and do more powerful attacks.
BlackStar's partner, Tsubaki, has a submissive and meek personality (though a ridiculously strong will), which reacts well to just about anyone. So her weapon form is easily wielded by anyone who gets her permission to wield her. Mind you she has five weapon forms she can shift between at the start of the series before soul resonation. Where as most weapon people only have one that only changes through resonations, but is still basically the same weapon.
Conversely, because of Black Star's personality, he can barely use any weapons, but he can attack people with his soul wave (kinda like a Chi punch).
Weapon people can partially "wield" themselves. Soul Evans for example turns into a scythe. He can summon a scythe blade from his body (usually his arm by transforming said arm into the blade) and use that to attack people.
Another character, who can turn into a bloody chainsaw, can get chain treads on their body and use them for attacks.
So if say we dumped Amethyst into the SE-universe, she'd probably be able to transform into a multi-headed, jewel encrusted whip and be wielded by a Meister, or turn her arms into the whips and lash at people with them.
Other characters can resonate too, and I think it's implied as you get older, you can resonate easier with more people. But the main characters are students, so they kinda stink at it.
Later on, the characters do a bunch of "connected" soul resonations that act as an all around party buff for everyone resonating.
Noticeably BlackStar sucks at it, and pretty much all his combinations with Tsubaki have to be beaten into him (almost make him die) before he can get them right.
An instance of a soul nearly overtaking another is Crona and Ragnarok. Crona (no gender ever given so get used to seeing the name) is super meek and submissive pretty much to the point of being mentally broken. Barely any willpower, and is constantly bullied by Ragnarok. Who is literally Crona's blood, which is an experimental "Black Blood of Insanity" that gives Crona EXTREME power, but Ragnarok as a partner (Not all people with black blood get Ragnaork as a partner...Crona is...a unique case to keep it none-spolierish). Raganrok bullies Crona to the point if you look at their souls, Ragnarok's pretty much eclipse Crona's to the point it's hard to even see it's there. They resonate just fine a Crona is pretty much subservient to Ragnarok. And Crona never tries wielding other weapons. though due to the black blood, it's implied that Crona's madness would spread to anyone Crona tried to wield, so most probably wouldn't try. Soul (due to getting black blood later) and Tsubaki (due to her adaptive personality) might have been wieldable, but we never really find out.
A full case of a soul eclipsing another is Asura and his partner Vajra, who Asura (God of Paranoid Madness) literally ate Vajra. He can still use his partner's abilities (beam firing his mad soul's resonance at people), but we never hear from Vajra, not even once, or see him transform into a human form.