As a new Owner, I have this Issue too. I can start the right Engine but not the left one. No Difference in Startup Sequences or Button Settings. Fuel Flow but no Ignition and the the Engine won't start. Master Caution goes on and the "L SRL Fail" appears. Ignition Cont Switches are set to Auto.
I sometimes have this Issue with other Planes too.
And one thing. It's a Turbo Prop, so Mixture Levers should have no Effect. From my copied Bravo Profile, the Mixture Levers were still set with Mixture Control and Fuel Cut Off with the Detent Switch. It HAS a Effect. I was not in Fuel Cut Off but going from Rich to Low Mixture cuts off the running Engine. I deleted all Bindings for that but the left Engine will still not work. Hmm, someone sold me a broken Plane.
Okay, after trying another Plane and reloading the MU-2, none of the Engines won't start. Restarting the SIM does the Job and I'm on my first flight. I also think there is a bug in XP12 with the Engine Types.
Very nice Plane indeed.