Bridged outputs are always Mono
When bridging channels together, the resulting output is a mono output created by summing the left & right input signals together. However, if there are 2 zones outputting to bridged speakers and you want one to by the Left signal and the other to be the Right, we need to get a little creative with our design.
The standard bridged design
In systems with multiple bridged outputs, often an installer will connect the left & right RCAs from the source to a BUS input and call it good! That will work and power all speakers equally. And, even though it may be providing a stereo signal, we know the resulting output will be mono.
The Stereo Bridge
To create a stereo image, take the Left RCA signal and plug it into the input for 1 zone, and take the Right RCA signal and plug it into the other zone.
For example, on this Director Zone 4 (7-8) and Zone 5 (9-10) are bridged mono outputs. The left signal is going into input 7 and the right signal is going to input 10.
In the amp's web interface we set both zones to Mono so both amplified channels become active. Now Zone 4 is playing the left signal and Zone 5 is playing the right.
If multiple outputs need independent left and right channels on an amp like the Director D4600 or Architect P2660, you can use the BUS inputs. The left signal can go into BUS A, the right into BUS B, and then use the web interface or dip switches to assign the outputs accordingly.
At this time, Digital inputs' stereo signal cannot be split between multiple outputs. A digital signal will only output as a combined left/right mono signal to bridged outputs.
Further Reading