Yes, it's possible.
By creating an aggregate device, you can combine the audio interfaces of the dual Apple LED Cinema Displays into a single speaker.
Here's an overview of the process:
- Open /Applications/Audio MIDI Setup
- Click + to add a new Multiple Output Device
- Select one or more Display Audio devices
- Adjust the volume slider for each Apple LED Cinema Display
There is one caveat to this setup — you can't adjust the volume using either the keyboard shortcut or the speaker icon, as it appears disabled in the OS X Menu Bar†. you can Option + Click to toggle between audio output devices though.
Sunflower is an open source third-party app that will give you the volume control back, but since it's a kernel extension you should understand the implications before installing it.