That's not how I read the manual:
"10 - 2TK LEVEL
The rotary control sets the level of the 2 Track Tape input, which is routed to the headphones, monitor outputs and meters, or directly to the Mix outputs by pressing the adjacent switch (12). These unbalanced inputs, on RCA phono connectors, are ideal to connect the playback of a tape machine for monitoring.
11 - 2TK REPLACES MIX OUTPUT!
This switch does what it says! When pressed the Mix output is switched to the 2 Track input and is an ideal way of feeding pre-show music to the main outputs in live applications without using up valuable input channels. For example, with the switch pressed and a CD connected and playing pre-show music, the engineer can be setting up channel levels, adjusting EQ, talking back to performers and monitoring any part of the mix (including final Mix) without affecting the sound to the audience. Releasing the switch instantly swaps the Mix Outputs back to the mixer and cuts off the CD.
CAUTION: Pressing this switch cuts off the normal Mix L/R signal and it should therefore NOT be used during live performance or recording"