The spirits I also feel are a non-issue because of (Fighter). However, iirc, with Pokemon Trainer, the Trainer and each of the Pokemon have their own fighter spirits. I feel like if both Pyra and Mythra were included, they would go that route and if it were just Pyra, then Rex & Pyra together.The announcer calls out Rex & Pyra or Rex & Mythra depending on whoever’s out. You can pick which one to start with a la Pokémon Trainer.
If Minmin has shown anything it’s that this sort of thing is a non-issue. Hell, their Spirit could just be Rex & Pyra, and Rex & Mythra without the need for (Fighter) appended because they’ve never shared a Spirit before anyways.
However, the difference between the spirits and the character select screen is they'd have to have Xander Mobus/Announcer in every language record two lines specifically. Also not impossible (Bowser Jr., though that was base game), but I'm sure it's not ideal.
I also think this would probably be easiest, but as a fan of Pyra and Mythra...feelsbadman lol. As fleshed out characters, you sort of want to see them get acknowledged compared to a generic Pokemon but I think it comes down to how important Pyra and Mythra are in Rex's moveset.Honestly I think just calling them Rex would be the easiest. Especially if they are intangible. There is precedence for this as the announcer calls out for the trainers rather than the individual Pokemon.