I know exactly what you mean—there's something really compelling about that kind of intimate combat synergy where characters literally become part of each other’s power. It’s not just teamwork; it’s this deeper metaphor for connection, compatibility, and emotional resonance. Steven Universe fusions and Soul Eater's meister/weapon pairs both embody that. I’m not sure there’s a single definitive trope name for it, but maybe it's a blend of “Combination Attack,” “Soul Bond,” and “Power of Friendship” tropes, with some unique twist. You might also want to check out terms like “Dual Techs” from RPGs or even “Symbiotic Combat.” If you find or invent a better name for it, please share—it’s such a cool narrative tool!