Streaming to AVANTREE LEAF device.

I have an AVANTREE LEAF USB stick which acts as a sound card for streaming music to Bluetooth speakers. It is plugged into a USB port on my Win10 PC. I have 2 x Amazon Echo speakers, lets call them Hall (H) and Kitchen (K). I pair H to the Leaf and a speaker selection appears on the Win10 PC and I can then play music on H. I then pair K to the Leaf, no additional speaker selection appears on Win10, the one that previously appeared for H now plays music on K and I can no longer select H to play music. It's notable that when each Echo "sees" the Leaf it identifies it as a MAC address since the Leaf acts as a sound card.
So now I wonder... Is this a Win10, or LEAF, or ECHO problem? Does anyone have any experience?

I, of course, hoped that I would be able to select, from the speaker selection on Win10, either of the Echo speakers for playing music, but the result now is only one speaker can be paired, and thus only one play music. If I wish to play on the other I have to alter the pairing.

EDIT:
The support page confirms that the Leaf does not support the simultaneous pairing of more than one Bluetooth listening device. Maybe because it uses the default generic MSoft audio driver.
There is another problem with this device when pairing an Echo. After pairing it works fine for music streaming. However, when you revert to normal Echo use, for example listening to BBC Radio, then the broadcast is cut after around 10 secs. This persists until the Leaf is removed physically from the Win10 PC. Considering the cost it's not the best buy for working with Echo devices. It reaches further than any other Bluetooth dongle I've experienced; paired with an Echo 25 ft away through two walls, but I'm tempted to return it.