You only need to connect the CD player to one ERA100 as you already have, once paired with another ERA100 the stereo signal is automatically sent to the correct speaker.
The Line-In adaptor is a stereo device. When playing through a single speaker, both channels are played in that speaker, however, you can “Bond” another ERA 100 and form a stereo pair.
Note that Line-In is simply another music source on the network that can be played (“Grouped”) through any combination of Rooms. A Room can be a single speaker, pair, or surround. One point of confusion is that Line-In is not automatically played through the speaker that is hosting the physical Line-In connection.
Thank you all, that definitely answers, and no need for second Line In/RCA cable, cheers.