LarryR,
The two-page instructions that came with my 1986 MI 100-8 say under "Additional Instructions For Two Channel Model": "The 'mode' switch provides stereo capability with the addition of a slave amp and speaker. To achieve separation using different signals, set the 'mode' switch to "STR" (stereo). This separates the line outputs for channels 1 and 2. Next, using a shielded phone (instrument) cord, 'patch' from either line output (channel 1 and 2) to the adjacent 'line input'. Finally, using a second instrument cord 'patch' from the remaining 'line out' to the slave amp input. When stereo operation is not desired, set the mode switch to "MON" (monaural)." Hope this helps.
On an unrelated matter but for anyone (at least those using or contemplating using a mid-80s MI 100-8 or MI 225-8 WW amp for both electric and acoustic bass) who might be interested, the instructions also say "Unless otherwise specified, channel one equalization is different from channel two. This was done to give greater sound control flexibility. However, most players feel that channel one is preferred for electric bass and channel two is preferred for acoustic bass."
Finally, one other item of possible interest to those fascinated by WW lore is that when I purchased my MI 100-8 in 1986, it came with a complimentary red tee-shirt with a small "Walter Woods Amplifiers" on the left chest and a large "WW Amps" logo from that era on the back. Realizing at the time that it was probably not easily replaced, I've never had the nerve to wear it (which sort of defeats its purpose as either a garment or source of advertising).