Hello @mejki
You CAN connect multiple devices with a little trick 
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Disconnect all devices of your flow even the one where it says "[device name] (This device)".
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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.
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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 