@Timothee-Leblond I have a couple printers with mixing extruders. In your last example with a mixing ratio of 0.02:1 Your printer will take the E value from the running gcode and you will get a total extrusion of 1.02 from a single nozzle. Not much different than setting the extrusion multiplier in the slicer to 1.02 on a normal printer. You might want to keep the total of all the extruder motors = to 1.00 when you add them both up in your M567 so that your commanded total extrusions are exactly what the running gcode specified.
If you have set the E-steps correctly on each motor then total extrusion should be accurate to what is commanded in the running gcode.
You can set M567 0.02:0.98 in the gcode file, in a macro, or in the command line as the print is running and it will change.
I don't recommend a setting one or the other motor at 0.00 as the motor at 1.00 will try to push some filament back up the unused side and could cause a blockage similar to heat creep.
I set both extruders to 0.50:0.50 in config.g just in case I accidentally send a print job without specifying the mix ratio.
; Tools
M563 P0 D0:1 H1 F1 S"Dual" ; define tool 0
G10 P0 X0 Y0 Z0 ; set tool 0 axis offsets
G10 P0 R0 S0 ; set initial tool 0 active and standby temperatures to 0C
M567 P0 E0.50:0.50 ; set mixing ratios for tool 0 Dual
Geoff