Actually, the 1957 did the same job (s) with a single mercury switch. The dual setup on 1958 models allows a slightly different make/break time for the different jobs. Is there a good reason not to use a single switch on a ’58? Not that I know of, if you adjust it properly. Just use the ’57 wiring diagram if you elect to do it that way.

BTW, later on they went back to a single switch to do both jobs, but used a micro switch instead of a mercury switch. Micro switch was a big improvement.