Quote: "Does it stream the audio to the main computer aswell so you could turn on a youtube video or something on the secondary and use the same headphones?"
No. The sound is played on the system it is opened on.
Synergy (and Input Director) allow you to "jump" from your main system to the other defined system and launch apps on it. It is handy if you have a dual monitor setup on a desktop and need a third screen for something else to be open. We use both Synergy or Input Director on our network using two desktops with dual monitors and two laptops. So if I am using the desktop to code in FPSC or DBP, I may use either Synergy or Input Director to jump to the laptop screen to open my design doc in word or launch Photoshop and my script editor to do my huds and scripts while designing my level(s).
There's no problem that can't be solved without applying a little scripting.