i feel your pain about splitting of audio without a headphone jack. there are some devices like the Airfly Duo (no affiliation) that can connect via Bluetooth to two devices, but those require that at least your device has an audio jack
IIUC, this is part of Bluetooth v5, it might specifically be connected to LE Audio, but I don't know the details.
"Multi-Stream Audio will enable the transmission of multiple, independent, synchronized audio streams between an audio source device, such as a smartphone, and one or more audio sink devices"
https://www.bluetooth.com/learn-about-bluetooth/feature-enha...