This is what you are looking for (make sure to read the important note at end of this post though!):
Onvian 6.35mm Male to Dual 3.5mm Female 3 Pole TRS 1/4" to 1/8" Audio Adapter Convertor Y Splitter Cable
From the description:
✅1/4" 6.35mm Stereo Plug / Male to Dual 1/8" 3.5mm Stereo Jack / Female Splitter Adapter Cable 0.6 Feet (20cm) 6.35spf-2/3.5spm
✅Allow you to connect two headphones or sets of speakers (which use 3 Pole 3.5mm jacks) into a single output splits any 3 Pole 3.5 mm audio jack into two connections.
If your output needs a 3.5mm plug, then you may also need this:
3.5mm male Plug to 6.35mm female Stereo Jack Adaptor
Note: This will not make one speaker "Right" and the other speaker "Left" however. Each JBL Charge has two speakers in it, one left and one right, which is why each Charge wants a stereo signal. It is possible to make one JBL charge only receive the right signal and the other only the left signal, but it will require a more complicated setup.
I recommend just sending a stereo signal to each speaker (which the cable above will allow you to do) and see how it sounds, if it doesn't sound good then we can talk about the more complicated setup.