HOW DO I DOUBLE UP WONDERBOOM 3 SPEAKERS TOGETHER

Long press on the primary speaker center button which is playing the music. After hearing the sound effect of the primary speaker (music stop), then long press on the secondary speaker center button. To enter the true wireless stereo mode, press the center buttons of those two speakers at the same time under the double-the-sound / party louder mode, you will hear this confirmation tone. You could switch between 1st mode and 2nd mode by simply pressing two center buttons again.