A+ driving multiple speakers

Hi,
I would like to make A+ play through MULTIPLE speakers connected to my iMac by usb and/or Bluetooth.
I tried everything. Keep getting the “hogged by another application” message. Has anyone found the solution?

Did you check this:

https://support.apple.com/en-us/HT202000

Combine multiple audio interfaces by creating an Aggregate Device

Yes, checked and tried, doesn’t work with A+