How do I connect two PartyBox 310 speakers in TWS (True Wireless Stereo) mode?

Press the TWS button on both speakers to connect them to each other in TWS mode. By default, the speaker connected to your Bluetooth device acts as the primary speaker (left channel), while the other acts as the secondary speaker (right channel). To exit TWS mode, power off or press the TWS button on either speaker. (Page 14)