I think the way audio routing works prevents this.
I have speakers which connect to other speakers. It's neat, but the chaining is done with some app, it doesn't work "naturally". It's also the device being able to chain.
There are multipoint headphones out there though I think phones must enable support. These headphones connect to other headphones with two different power modes, it isn't the phone outputting two things at once.
Hardware splitters exist.
Though without a headphone port. I dunno. You'd need the charging port adapter.