Soundcore 2 True Wireless Pairing

Does anyone know how to pair two “Soundcore 2” devices using “True Wireless Pairing” so that they can act as stereo output for a single Bluetooth input?

I have two devices, they are both definitely the “upgraded” model (A3105014, their serial numbers are 6 apart). One of the devices is paired with my PC. The instruction manual is utterly useless, but I found random Google advice to hold down the Bluetooth button for 3 seconds and the Bluetooth light will double-flash to indicate “True Wireless Pairing” mode - do this on both devices and then the instructions just stop which I assume means “it should be working now”.

Holding down the bluetooth button on the speaker which is not paired with the PC does result in a double-flashing light, but on the other device that is paired with the PC, no amount of pressing/holding the bluetooth button will make the light double-flash.

I tried un-pairing the device so they were both un-paired, then holding down the bluetooth button, this does result in both devices double-flashing, but then nothing else happens and after a while both devices leave this mode and stop double-flashing.

I tried to update the firmware on the devices in case that would make a difference, but Anker appear to have removed every single firmware file from their web site except one for the “Soundcore Boost”!

Some other guides may be helpful

I would do the following.
Do not connect the speaker (master) to the PC.
Best is to turn off the PC so there is no connection possible and then try to pair the speakers in the upside told way.