Hi
This problem seems to be only with IOS based devices. I tested this with iphone, ipad and Samsung Galaxy A3.
Instead of your speakers I have two JBL charge 3 speakers, but have the same problem.
To understand connections by bluetooth better, I renamed by JBL Connect app the speakers as JBL1 and JBL2. This is not necessary if you have different names already.
The solution is this:
Connect iphones bluetooth only with ONE speaker so that the power button of the connected speaker will change blue. The other speakers power button will stay white.
Go to JBL Connect app and after you see the connected speaker push the orange JBL Connect button and just after that push the Connect button on both speakers.
JBL Connect will do the pairing.
Select either party mode or stereo mode.
Play the music, increase or decrease the volume or do anything else, it will work.
If you now have a look at the bluetooth settings of iphone, you will see the connected device twice on the device list.
If you connect also the other speakers bluetooth connection on, you will have the problem back.