Shuttle had this; it had five onboard General Purpose Computers (GPCs).
For dynamic flight phases (launch and landing) four of the GPCs ran the Primary Avionics Software System (PASS) coded originally by IBM, later by USA.
The fifth GPC ran the Backup Flight System (BFS); a bare-bones implementation designed just to get you safely to orbit or landing, coded originally by Rockwell International, later by Boeing. To keep it from being completely unused on nominal missions, it also performed some Systems Management (SM) functions, like turning cooling systems on, and had some crew displays of SM information.
The BFS monitored the PASS via the flight critical data buses so that it acted as a "hot spare", ready to be engaged instantly if needed.
The BFS was not used nominally in orbital operations. Should a complete PASS failure have happened on-orbit, the BFS had the capability to get the Orbiter back.
Since the BFS, if it were ever to be used, would probably have been when the Orbiter was going out of control, it was simple to start. Just mash the red button on either hand controller; that killed the PASS and engaged the BFS.
(personal photo from Shuttle Mission Simulator)
One could demonstrate their newbie status by mistaking this for the push-to-talk button.