I believe belezeebub is right about this space. Attached I have a low-res image I dug up from an Australian car sales site.

On my US-built 98 Civic Coupe, I have a similar space down there, which was originally intended for the factory 6 disc in-dash CD changer. However, I have used that space to mount a normal DIN sized CD player with an AUX input.

Instead of using the manufacturer supplied "sleeve" to mount the CD player, it is instead directly screwed to the frame of the dashboard (ISO mount?), as I believe the factory changer would be. The holes in the CD player and the dash frame lined up perfectly. I originally tried to mount the empeg's sled there, but the space is a bit too wide and short to be called DIN. Sorry I don't have any pictures to give you a better idea.

It looks to me as if you have some extra plastic bits down there, so I'm not sure if what I did would be possible in your car. But given that they are so similar, I wouldn't be surprised if you were able to put a second head unit there using the same method. Do you feel like taking apart the dash to find out?