You CAN connect multiple devices with a little trick
Disconnect all devices of your flow even the one where it says "[device name] (This device)".
now first connect your smartphone to the flow of your main pc and write a test message like "Test" to make sure everything is fine.
now take your smartphone again and scan the qr code of the 2nd device that you want to use. Again write a text message like "Test" to make sure everything has worked fine.
Its not a feature but a bug i think. But cause of this bug you should have 3 devices in your Flow now