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
Splitting audio via Bluetooth works completely fine for me. Doesn't need any extra apps or devices. But I was also very surprised when found out that this works now.
I went searching for it the other day, and my impression for most tablets/phones is that it's supported at the hardware level by most modern devices, but it needs to be enabled at the OS level. Right now, only Samsung are really doing that, but presumably at some point Android and iOS will have it as a built-in feature as well.
IIUC, this is part of Bluetooth v5, it might specifically be connected to LE Audio, but I don't know the details.
Ok, I found out now that the feature is called "multi-streaming".
"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"
