It is quite bad that they chose the term "ECU fail" for the warning.
It is very rare for the ECU to fail and would be even more rare for a double ECU failure.
The warning seen by the pilot just indicates that one or more of the parameters monitored by the computers has gone outside the limits programmed.
In the post above about an engine shut down with a double ECU fail was (informed guess here) probably a prop valve failure which would cause the engine to feather. The ECU fail is simply reporting the fact that oil pressure etc is outside parameters and not that the computer has failed.
The map pressure is monitored by both computers with seperate hoses from the engine, if one of these was to develop a small leak, the computers would identify the difference and display a "ECU A (or B) fail" warning, it would not affect engine operation and a similar fault would probably go unoticed on a regular engine for some time.
Not draining water from the drains can also generate a double "ECU fail" once again, the computers have not failed it is just registering the fluctuating fuel pressure as water passes through the system.
One aircraft that was returned to me with a double engine, double ECU fail (all 4 computers showing fail warnings) was rectified by draining over 2 ltr of water from each wing.