This depends to some extent on what you mean by a "major" US auto manufacturer. There were more than two US auto manufacturers that went out of business or merged during this time, so it is not clear which two you will want to mention.
I would say that the two most famous auto manufacturers to merge or go out of business were Packard and Studebaker. These two companies merged in 1954. By 1958, Packard had stopped producing any cars. Studebaker made its last car five years later, in 1963.
These were two of the more famous auto makers in the pre-war period. Both had been in existence since the earliest days of the 1900s and had produced many famous cars. The demise of these firms was a major example of the consolidation of the auto industry after WWII.