Method: Using an AUX Splitter
What You’ll Need:
A 3.5mm AUX splitter (also called a “1-to-2 audio splitter”).
Two 3.5mm AUX cables.
Your phone or MP3 player or other audio device (with a headphone jack or a suitable adapter).
Steps:
Connect the AUX splitter to your phone MP3 player or other audio device:
If the device has a headphone jack, plug the AUX splitter directly into it.
If it doesn’t have a headphone jack (e.g., newer iPhones or Android phones), use the appropriate adapter (e.g., USB-C to 3.5mm or Lightning to 3.5mm) to connect the splitter to your phone.
Connect the speakers to the splitter:
Use two AUX cables to connect each speaker to one of the two output ports on the splitter.
Plug the other ends of the AUX cables into the AUX IN ports on each speaker.
Set up the speakers:
Turn on both speakers and set them to AUX mode.
Play music:
Open your music app on your phone and start playing music. Both speakers should now output the same audio simultaneously.

Please see the photo below.

Please see the photo below.

Our soundbars could be controlled through panel buttons & remote controller, supporting Bluetooth connection & AUX in & USB playback. It does not have AUX out port.

Unfortunately, there is no memory function available for saving the settings when the soundbar is powered off. So once the soundbar is powered off and on again, you need to reset the setting.