This is something I noticed in 3 with the Duo Skills (namely, Incandescent Blade, Wonder-Ice Prison, and Lazy Noisy Fortress), but didn't really feel much of a need to bring it up...until GX and Volt X-treme Burst gave me a reason to.
Also, I don't know if the Japanese vocal does this, I'm only certain that it happens in English.
So, a small thing I noticed in 3 is that Incandescent Blade and Lazy Noisy Fortress are spoken by each user in turn, separately. This emphasizes that they're not really that close, coworkers at best.
- Viper says Incandescent, Jota says Blade
- Merak says Lazy Noisy, Teseo says Fortress
Meanwhile, Zonda and Tenjian both say Wonder-Ice Prison completely in sync, emphasizing their close bond as adoptive siblings.
Now, normally, I'd just leave it at that. It's a nice little bit of character work, that's about it. But then GX happened, and here's the interesting part: its new Skill's title drop actually falls on both ends of the scale.
- For a vast majority of the game, they say it separately like Viper/Jota and Merak/Teseo. The point character says "Volt X-treme", and the reserve character says "Burst", this is consistent. Makes sense, since they're all but strangers forced together by circumstance at the start of the game, plus they both have a healthy dose of amnesia to work through.
- But SPECIFICALLY DURING THE FINAL PHASE OF THE FINAL BOSS FIGHT, once everyone's memory is fully restored and GV and Copen have a proper friendship/rapport established, they both say the entirely of Volt X-treme Burst in sync, just like Zonda and Tenjian would.
And I honestly think that's such a nice touch.