Since I do not have 2 PCs readily available, I'm not sure that this works. But by experimenting around this solution you should be able to get it to work.

Let's say you want to work from PC1 and play sounds on both PC1 and PC2.

On both PC1 and PC2:

Install PulseAudio Preferences:

sudo apt-get install paprefs

On PC1 only:

1. Open PulseAusio Preferences dialog (from Dash, say).


2. In the Network Access tab enable Make discoverable PulseAudio network.... This will make the sound device in PC2 accessible from PC1, if allowed by PC2.

On PC2 only:

1. Open PulseAusio Preferences dialog.


2. In the Network Server tab check Enable network access to local sound devices and Allow other machines on LAN to discover local sound devices. This makes the sound device in PC2 visible to PC1.


3. Do not check Don't require authentication - that will allow others on the LAN to spam your sound system.

Providing authentication:

Copy ~/.pulse-cookie file from one of the PCs to the other, so that that file is identical in both PCs. It doesn't matter which PC's file is copied to the other.

At this point (possibly after rebooting the two PCs), you should be able to play sounds from PC1 on PC2's sound system:

On PC1

1. Open the volume control dialog.


2. In the Output tab change to PC2's sound device (Under the list "Play sound through").


3. Play some sample audio in PC1 and verify that you can hear the output in PC2.

Now you want the sound server on PC1 to forward sounds to both sound devices (available locally to it). For this you have to create a virtual sound device whose slaves are the two available devices. An easy of doing this is

On PC1:

1. Open PulseAudio Preferences dialog.


2. In the Simultaneous Output tab, enable Add virtual....


3. From the volume control dialog, choose this virtual output device.

Notes:

1. Simultaneous output on all local sound cards may not be desirable, in which case you should take a look at pacmd's module-combine option to combine only select sinks.


2. If you wish to work from either PC1 or PC2 and want all sounds to play on both PCs, perform PC1 and PC2 specific steps on both PC1 and PC2.


3. Hopefully the network is strong enough that no noticeable sync errors will creep in. If not, apparently one will have to modify some buffer sizes to set things right.

Sources:

1. http://blog.nekoconeko.nl/blog/2012/03/03/PulseAudio-over-network.html


2. http://psung.blogspot.in/2011/09/network-audio-with-pulseaudio-made.html