You can use a package called Jack. What it lets you do is pipe the outputs from the source, into whatever channels on whatever devices you want.
sudo apt-get jack2d jack-tools qjackctl
jack2d: is the more recent version of Jack
jack-tools: more stuff for jack
After installation use
qjackctl to pull up the GUI.
Examples of things you can do with jack;
- Pipe audio from one source to all devices for synced audio.
- Send the Left channel to both channels of a pair of speakers, and the right to a different pair
The possibilities are pretty endless. Here is a picture from the help page for Jack
On the left, you can see your output sources, and on the right are the devices you can output to.
Here is a picture of 3 LR audio sources where 2 of them are pipe to 1 speaker,
and the other source to the other speaker.