If that's the case, probably the USB connector is not able to supply enough energy to both miners at the same time.
Try connecting them to different USB connectors (although they might be connected internally).
They are both on the same powered USB HUB (Atolla), there are no USB ports on a MacBook Air, everything goes through one USB C port. I have a powered dongle with several different types of ports that plugs into the MackBook. The USB HUB is also powered.
Thanks anyway, it's a bit confusing with my Mac, I might pick up a PC with Windows 10 and try that as it looks like it that's more common.