Cannot connect multiple PS4 controllers through bluetooth
I successfully managed to connect 2 DualShock 4 on my Mac mini 2012 running High Sierra 10.13.6 using a Bluetooth Dongle (not with the builtin Mac mini BT controller).

Both gamepads could not be connected simultaneously as described in the question.
Purchased Mac-compatible Bluetooth USB dongle (https://www.amazon.fr/gp/product/B009IQB3US/ref=ppx_yo_dt_b_asin_title_o01_s01?ie=UTF8&amp;psc=1). You need to check for Broadcom chipset to be supported by Mac (no driver needed). Then voilà! Both gamepads connected / working at the same time.

Recommendations:


Delete all BT devices paired from Mac
Make sure Mac picked up the BT dongle as controller vs builtin one (ALT-click on BT icon and check for MAC address change).
Reconnect / pair BT devices
I had to reset one of my DualShock (https://www.lifewire.com/how-to-reset-ps4-controller-4174025)


PS: could not answer similar question Use more than one PS4 Dualshock controller over bluetooth due to lack of reputation

Hope it helps.