Possible to use two HCI controllers concurrently? I have a 2011 iMac with Bluetooth 2.1, and recently purchased a Bluetooth 4.0 USB dongle required for my new wireless mouse.

The dongle works great - I paired my mouse and keyboard to it without issue and used the following terminal command to ensure macOS uses the dongle instead of the built-in 2.1 module:

sudo nvram bluetoothHostControllerSwitchBehavior=always


(Source: Force OS X to use Bluetooth dongle instead of built in controller)

However I've noticed when I reboot or shutdown my Mac the login screen doesn't respond to any input from my wireless keyboard, so I can't enter my password to login. After logging in (using a wired keyboard) my wireless keyboard and mouse work perfectly fine. I suspect this is because I have FileVault enabled and the USB dongle's driver is not loading at boot?

It seems to me the best solution would be to leave my keyboard paired to my iMac's built-in module and only pair my mouse to the USB dongle, since I can login without needing my mouse, but I can't seem to find a way to use both Bluetooth modules concurrently. Is that possible?

Alternatively, is it possible to configure macOS or my iMac's EFI to load the USB dongle driver at boot? Replacing my iMac is not an option for me at this stage but I'm open to any other alternative ideas too. Thanks.
After some back and forth with Apple Support I've learnt that yes, macOS can use two Bluetooth HCI controllers at the same time, but the same driver cannot be used for multiple devices at the same time. As both my internal Bluetooth module and the USB dongle I purchased use a Broadcom chipset, macOS uses the same driver and that's why I cannot use both at the same time.

The solution is to use a USB dongle which does not contain a chipset made by the same manufacturer as the internal Bluetooth module. Broadcom seem to make the majority of chipsets used in most Bluetooth USB dongles - however I've found the 'Orico BTA-403-BK' which uses a CSR8510 chipset, which is manufactured by Qualcomm, who acquired a company called Atheros in 2011, whose Bluetooth drivers are natively supported by macOS. I'll report back after the Orico arrives with my final result.