The Bravo has 6 Levers. A normal Setup for 2 Engines (Piston or Turboprop) is 2 for Throttle (Black), 2 for Prop (Blue) and 2 for Mixture/Condition (Red). Some Turboprops use all 6 Levers, some not. The MU-2 uses Throttle Levers, assigned to Throttle 3 and 4 Axis, and Prop Levers, assigned to the Prop Axis. The Mixture/Condition Levers are unused and should not assigned to any Function. In Hardware, you can unplug the Levers and use the included Covers to close the Gaps.
If you copy a previous Setup for the MU-2 without deleting the Mixture Setting, it could cause in Problems with non starting Engines. I had this with Rotax Engines too. The confusing Part is, it happens not every Time and not for all Engines.
This is my Bravo Hardware Setup for the MU-2. Black Levers Throttle, Blue Levers Prop. And the Assignments in XPlane.