I have just added a pair of Era 300s to my Arc + Sub to replace the Ones I was using.
I have just tried a 7.1.4 test tone via an App on my Apple TV and when testing the up firing speakers both the front (Arc) and rear (Era 300) play at the same time when playing either the front or rear Atmos/ceiling speaker test tone (right or left). These should work independently right? Or do they operate in a .2 configuration rather than a .4 setup?