Here's how it works - there is a lot of soldering to do! 

Interface 
--------- 
First, stack the two SID chips together, leaving as much space as possible 
between them. 


On the second SID chip, bend out pins 1-4, pin 8, pins 23 and 24, and pin 
27.  Do not connect these pins to any point on the first SID chip. 


Solder all other pins of the second SID to corresponding pins on the first 
SID. 


On pins 1-4, attach two filter capacitors ("caps").  One to pins 1 and 2, 
the other to 3 and 4. 


If your 128-DCR has been modified to accept a 6581 SID, these will be 470 pF 
caps.  If not, you should use an 8580 instead (of course), and use a pair of 
22000 pF caps. 


Leave pins 23 and 24 alone, thier function is of no use in a Stereo-SID 
setup anyway. 


Addressing 
---------- 
Connect pin 8 to one of the following on chip U3 (a 74LS138): 


Pin 12:  $D700 
Pin  9:  $DE00 
Pin  7:  $DF00 


I recommend using $D700, as the other two addresses are often used by other 
devices such as your REU and Swiftlink cartridges. 


Audio Output 
------------ 
On pin 27, the last pin left to connect on the second SID, you will find 
the audio output. 


You'll need to hang a small preamp circuit off of this pin, routing the 
output of that pre-amp to the back of the computer as the final output. It 
consists of a 2SC1815 transistor, four resistors and four capcitors. 


It is the small block of circuits found next to the 6581, connected to pins 
14 and 27 of the SID, in the C64 and C128 schematic diagrams, which can be 
found in the back of your Commodore 64 and 128 Programmer's Reference 
Guides. 


If you don't have the schematics available, you should be able to find them 
on ftp.funet.fi.  You will need to use a machine with high-end graphics 
capabilities to download and print the schematics.