Likely because the developers were uncertain about double battles to begin with. They wouldn’t want one of their core recurring trainer cards to remind players of a discontinued format, and they wouldn’t want to errata an existing card twice, so they just created a newly named card to quarantine the dual-pokemon effect to something that likely wouldn’t be eternally printed.
Walrusaurus
Why didn’t they just errata Full Heal instead of printing this? There is no reason to play Full Heal over this in any deck.
Not that anyone would actually be running either of them, of course.
feyblade
Likely because the developers were uncertain about double battles to begin with. They wouldn’t want one of their core recurring trainer cards to remind players of a discontinued format, and they wouldn’t want to errata an existing card twice, so they just created a newly named card to quarantine the dual-pokemon effect to something that likely wouldn’t be eternally printed.