When using two Symphony I/O MKII Thunderbolt units you may run into this question: How do I hear my Low Latency Mixer of both units?
First let's look at the signal flow of a Multi-Unit setup.
You will see in Symphony Control that each unit's Physical Outputs can only source from that unit's Internal DSP Mixer. This is an internal physical connection.
So for Main Output, of Unit A, it's first two Out channels, our options are only A Mixers, same unit.
If you click on the B HP, headphone jack on Unit B, you have sources of B Mixers, same unit.
Thus, we need to take a feed from Unit B's Mixer and get it into Unit A's mixer!
How do we do this? Digital!
Each unit will have a SPDIF option, we can use for a Stereo feed of the Mixer Output.
In my scenario my Unit A has an 8x8 card, so it has 8 Analog and 8 Digital channels. My Unit B is a 16x16, so it has 16 Analog channels.
SPDIF on these cards can Mirror an Output or Replace an Input. Left panel, we will make adjustments after.
Working through the Signal flow, I have a signal generator going to Input 1 on Unit B.
You can see in the Mix Page, I have Input 1 of Unit B passing through to the Unit B Mix 1 Master.
Then in the I/O Page I route Unit B Mix 1 to Unit B Main Out. You can see it shows up on the meter.
Next, in the sidebar, I tell Unit B to have SPDIF Coax Mirror Analog Outs 1-2, and I tell Unit A to have SPDIF Coax Replace an unused Input of my choice. Here I'm going to use Optical 7-8, as these are unused.
VERY IMPORTANT, You use a SPDIF Coax 75 ohm cable from the Unit B SPDIF Coax Out to Unit A SPDIF Coax In.
You can see my signal is routed through SPDIF cable, to Unit A Mixer now!
And it now comes out at the Main Output of Unit A, as that Output is set to A Mixer 1.
Of course, this can be done in reverse, and even more complex routings. If you wanted to send Unit A's Inputs to Unit B, so you can also have full Monitoring from Unit B's outputs, you would do the same in reverse, and use a second SPDIF cable.
NOTE - you ARE going to have to REPLACE 2 of your Inputs to do this. So you would lose access to those for other inputs. If you are in a situation where you simply cannot spare 2 inputs for this in your whole setup you may have to use Software Monitoring and be subject to latency. But for those with large systems, 64 channel rigs, you should usually find you can spare 2 inputs on Unit A for this purpose, or vice versa, 2 inputs on Unit B, and route to your headphone boxes from that Unit.