This comes up often enough to warrant a bit of clarification
The same reed switch speed sensor can be used for both the m-Unit and speedometer (see Diagram below) but when you do this you have effectively split the signal diminishing the input provided to both the gauge and m.Unit.
TROUBLESHOOTING: depending on the order in which you set these up and operate the bike is possible for the m.unit to create an inaccurate baseline for the signal input. Should you find that you find that one or the other gauge reading is off you should follow the instructions in the m.unit manual for recalibrating the signal and voltage inputs for the m.unit followed by reprogramming the gauge if necessary.
This may or may not work for other gauge brands that use a reed switch speed sensor.