The good news is that now, the plugins are clearly separated. When I play a note on channel 3, the first plugin doesn't receive any data, that's good.
The bad news is that channel 1 is still used I don't know why.
As you can see in this example, the note off is send on channel 1 instead of channel 2. What the hell?91 2F 62 Ch.2 Note On: B1 (47), Velocity: 98
80 2F 40 Ch.1 Note Off: B1 (47), Velocity: 64
91 30 50 Ch.2 Note On: C2 (48), Velocity: 80
80 30 40 Ch.1 Note Off: C2 (48), Velocity: 64
91 34 50 Ch.2 Note On: E2 (52), Velocity: 80
80 34 40 Ch.1 Note Off: E2 (52), Velocity: 64
91 29 45 Ch.2 Note On: F1 (41), Velocity: 69
90 2B 3C Ch.1 Note On: G1 (43), Velocity: 60
80 29 40 Ch.1 Note Off: F1 (41), Velocity: 64
80 2B 40 Ch.1 Note Off: G1 (43), Velocity: 64
I'm gonna do a General Midi Mode Reset, to see if it changes something.
edit : doesn't change anything.