Beyond "Confusing" Chained Channel Map Modifiers

[indignant.ai]

Trying to simplify this for anyone that may want to help.  Trying to control two separate MPE (1 MIDI Channel per note) synths independently from a keyboard split.  MIDI channel 1 is inaccessible on both synths

Modifiers Used

CM1
In -> Out
1 -> 2
2 -> 3
3 -> 4
4 -> 5

CM2
In -> Out
5 -> 6
6 -> 7
7 -> 8
8 -> 9

CM3
In -> Out
9 -> 2
10 -> 3
11 -> 4
12 -> 5

CM4
In -> Out
13 -> 6
14 -> 7
15 -> 8
16 -> 9

Port 1 In from Keyboard split down the middle, half the keys going to Ch 1-8, the other half going to Ch 9 -16.
Port 1 In is going to Port 1 & 2 Out
Port 1 Out is getting modifiers CM1 -> CM2 (Going to First Synth, MIDI Ch 1 unavailable)
Port 2 Out is getting modifiers CM2 -> CM3 (Going to Second Synth, MIDI Ch 1 unavailable)

If I set the exclusivity of modifiers to CM1 = N, CM2 = Y, CM3 = N, and CM4 = Y, I get 5 out of 8 notes going to the first synth correctly and 4 out of 8 notes going to the second synth correctly.

Even if you can't help please, someone make that make sense.  Truly baffling...