It is possible to pair two of the same model of JBL PartyBox speakers together through TWS (True Wireless Stereo) mode.

PartyBox On-The-Go*/On-The-Go Essential*/100:
Connect just the left PartyBox with the chosen device first, with the right PartyBox turned off. The speaker that is connected first will automatically be the left channel. Now turn the right PartyBox on and continue with TWS pairing (detailed below).
*The PartyBox On-The-Go and PartyBox On-The-Go Essential are compatible with each other in TWS mode.

PartyBox 200/300/1000:
A button (L/R) on the rear of the speakers is used to assign the correct channel to the speakers after pairing is made.
In order to activate TWS mode, press and hold the Bluetooth button on the first speaker until the notification chime sounds, about 5 seconds. Then press and hold the Bluetooth button on the second speaker until the notification chime sounds (as seen below). TWS mode is now activated. To disable TWS, press and hold the Bluetooth button on either speaker, or power off one speaker.

Partybox 110/310/710/Encore/Encore Essential:
Press the buttons labelled TWS on both speakers to connect the speakers in TWS mode (pictured in item 2 in the image below, with two speakers on the button).

TWS can be activated and deactivated remotely in the JBL Partybox app, in addition to assigning the left and right channels to the correct speakers.
Once the TWS connection is setup, it will be in Stereo mode by default if both speakers are the same model, otherwise the speakers will be in Party mode by default if different Partybox models are used. You can also switch between Stereo and Party mode in the JBL PartyBox app if two of the same Partybox models are used.

Please note: In order to achieve TWS mode, the speaker must be the same model with the exception of the PartyBox 200 & PartyBox 300, and PartyBox On-The-Go & PartyBox On-The-Go Essential which can be paired together via TWS. You cannot achieve TWS with a PartyBox 1000 and a PartyBox 300. You cannot achieve TWS with a PartyBox 100 and PartyBox 200. However, it is possible to connect a maximum of two JBL PartyBox app compatible models (PartyBox 110, 310, 710, Encore or Encore Essential) together in Party mode - the two speakers then do not need to be the same model, but Stereo mode will not be possible.