Hey there pault,
what you want to do does work. I just tried it out with my ML10X and two synths.
The ML10X “simply” routes inputs to outputs. It does not know or care where that signal comes from or goes to.
In simple mode you could do what you suggested like this:
I used the main output as one output (i.e. your monitor speakers), but you could use any other loop as you suggested - it just makes more sense to me to use the output, because you don’t lose a loop just to send the audio somewhere and could still insert effects/something else into loop B.
You could place two stereo effects in loops B and C and send the synths to these independently. Anything before loop A would be affecting synth 1 and anything after it would be affecting synth 2.
The main inputs are fixed starting points in the simple mode though to answer one of your questions.
So it might not always be what you want.
If you want independent/more control over your synths (not having the main input in the path at all, turning them on or off independently, summing to the same output, sending to the same effect, naming them, completely independent signal flows …) you need two loops and use advanced mode. This should be able to do anything you want.
The drawback is that you can’t easily turn on or off loops with MIDI in this mode, so you’d need to create multiple presets for each added node you want.
That could look something like this. Synth 1 would the input, synth 2 in loop A return. The main output and loop E would be the outputs:
In the advanced mode you can also route multiple paths into the same loops:
As you can see the main Output nodes have multiple inputs but all other nodes can also accept two inputs.
so you could even send both synths to your DAW (as a mixed down stereo signal, unless you use another loop as another output) and
speakers at the same time, depending on how you connect it in the preset.
I haven’t tried how the two synths sound merged together, but it should sound fine with the main outputs at least. The other loops behave differently if sent two signals (see the second screenshot) - you’d need to try that yourself.