This gives a little more information on that drive.
http://www.atarimuseum.com/computers/8BITS...00/815/815.html
This is also listed in the 8-bit FAQ:
==> Atari 815 Dual Disk Drive --- dual 5.25" floppy disk drives in one unit
Were produced (all hand-built), but are very rare. One mode of operation.
Per drive:
1) Single-Sided, Double-Density--
MFM 40 tracks x 18 sectors/track x 256 bytes/sector = 180K capacity
19.2Kbps transfer rate. 288RPM.
The 815 includes a 6507 microprocessor.
Shipped with DOS 2.0D
MPI mechanism version- push button door opening for disk access
Tandon mechanism version- lift door, like a garage door disk access
accessories from Atari:
CX8202 Atari 810/815 Blank Diskettes (5 per box, certified for double density)
Stephen Knox writes (12/28/02):
I believe the story on the 815s was Atari didn't want to release them due to
severe QA problems with the drive but they had so many preorders they had to
release something. I think they filled the preorders and then cancelled the
model - Most of them got returned due to problems.