Jim,
If you look for a thread titled 99 Subaru Forrester (search for "Forrester gage" and you'll get only that article), you'll get a rundown on my install, which sounds similar to the two-DIN stereo you mention. I too wanted to use the stock stereo as the head unit (I had the 'premium' stereo option). Trouble was the cd/head unit interface was a proprietary format, not a standard RCA in/out. With some work, I'm sure it could be done, but I settled on getting a new head from Crutchfield.

The other thing you may face is that although the slots are DIN, the opening through the dash plastic is not, and I had to trim mine to get the empeg to slide in and out. My replacement tuner/cd (as slave) did not requre modifying the dash trim.

To answer your core question of whether or not you'll be able to fit two DIN units (empeg + another), I believe the answer will be yes.

-Zeke

just say you weren't paying much attention...