Normally you can only connect to 1 bluetooth speaker at a time using the normal OS X bluetooth and audio controls. But there is a way to join several speakers together so that they appear as one audio device. This means you can send audio to multiple bluetooth speakers at once.
Go to your Applications Folder and then to your Utilities folder.
Find the built-in OS X application called ‘Audio and MIDI setup.’
Press the plus button and select ‘Create Multi-Output Device.’
Select the tick boxes for the 2 (or more) devices you want to link, and they will be mirrored to each other.
If the devices are the same brand (eg 2 Marley Get Togethers) you will not need to click ‘drift correction’ but if you are using 2 devices that have different audio sample rates you will need to enable ‘drift correction’.
There will be a new audio device available in the System Preferences called ‘Multi-Output Device’
If you select this OS X will send the system audio to those various devices at the same time.