Jim
Sorry for no better answer, but with two EV3 bricks I have no problem using all 8 motors as I intend to do.
However I came to a lot of problems when using more than two ... for example my large LEGO Mindstorms Milka Chocolate factory (using 6xEV3) I had to split up in 3 x 2xEV3 ... but that worked without any problems!
3x 2xEV3 --> LEGO Mindstorms Milka Chocolate factory --> https://www.youtube.com/watch?v=VoU3Cmg-5eo&t=20s
2xEV3 --> Mr. Mixer Cocktail Robot --> https://www.youtube.com/watch?v=7mpUl5yWWWM&t=5s
cheers
Mike