I have been having this problem with my setup for quite some time now, and i hope to get some advice to get it solved.
I have a Connect, a play:5 and a Play:1. And when I want to play the same source on all at the same time, the Connect stops playing. There is no problem playing the same source on the two Play units, and it also ok to play radio on the Play units while playing an album from Tidal/Spotify on the Connect. But as soon as I try to play the same music on the Connect and any of the Play units, the Connect goes silent.
They are all hooked up to the same wireless network, from an Apple AirPort Extreme, with a 60Mbit internet connection.