I read about the Behringer MX9000 and found out that it uses the 8 subgroups in pairs:

Each input channel has:

Button1: Subgroup 1-2
Button2: Subgroup 3-4
Button3: Subgroup 5-6
Button4: Subgroup 7-8

While this works great with stereo instruments and an audio interface it becomes a little limiting if you connect mono instruments also. Take a look at this scenario:

I need to record:
3 synths (L + R)
2 mics

Channel1:
Synth1 L Button1 pressed

Channel2:
Synth1 R Button1 pressed

Channel3:
Synth2 L Button2 pressed

Channel4:
Synth2 R Button2 pressed

Channel5:
Synth3 L Button3 pressed

Channel6:
Synth3 R Button3 pressed

Channel7:
Mic1 Button4 pressed

Channel8:
Mic2 Button4 pressed

As you can see the button4 (subgroup 7 and 8) is routing to both mic 1 and mic2. Then they end up on the same tracks in the sequencer which is not what I want! Luckily the Behringer MX9000 has direct outs that can be used on the two mic mono channels. There might also be a way of routing one channel to only one subgroup, but in the manual I didn't find any such instructions, just 4 buttons with subgroup pairs enabled/disabled.