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.
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...