The dual-floppy modification was called the MacDouble D-D, it was two internal 400k floppy drives.
According to this page (link), the company would either do the install work on your machine (I assume you would provide the 2nd 400k drive), or you could buy a machine with both 400k drives for $1,995. The second image below which was found
in an old 68kMLA post specifies the $1,995 is for a 512k with dual drives.
All in all it's an odd modification. If it was shortly after the introduction of the original Mac it would make sense, but by July 1986 there were so many other options available. Including just an external drive.
Still, if anyone knows where one is, please PM me.
View attachment 65345
View attachment 65346