It is not possible to connect two of these Bluetooth audio transceivers to an iPhone 15 at the same time in order to use both on two different devices (stereo system and radio) in parallel in the sense of multi-room. Although the adapter supports multipoint connections in RX mode, i.e. simultaneous connection to two source devices, it does not support the simultaneous connection of one source device (e.g. iPhone) to two receivers for parallel audio output. The connection to each device individually works, but simultaneous use on two different devices via an iPhone is not intended with this model.

no batteries, it has to be plugged on an usb port (typically a hifi device with usb but no wireless or a car)

There's a USB connection when you lift the cap