I remember starting a paperback book my sister had when I was a teenager, in the late 80s or early 90s. It featured two aliens left behind on Earth. They become the basis for mankind's idea of the devil and god. I don't remember how old/new the book was.
My first thought is Waiting for the Galactic Bus by Parke Godwin (1988). It fits your distant memory to a "T". It features two brothers of a race that have extraordinarily advanced abilities, but still have a certain "frat-boy" attitude that leaves them stranded on primeval Earth. To while away the time (millennia), they "tinker" with the indigenous primates...
It featured two aliens left behind on Earth.
The tale begins with two college-age brothers, Barion and Coyul, members of an advanced alien world. Their race is endowed with the power to manipulate physical matter with their minds, a power which is exploited incessantly by the young adults. An accident strands the brothers on Earth, which at the time has no human race. The brothers hope for rescue, but eventually grow despondent. In their free time, they cause a series of evolutionary changes in the indigenous primates of Earth, which eventually lead to the blossoming of human civilization.
They become the basis for mankind's idea of the devil and god.
The brothers grow fond of their project, which they ardently monitor, intervening when necessary.
Subsequently, the two brothers literally put the duo through hell to keep them apart, subjecting them to outrageous scenarios beyond their control.
This may be a common trope; it is also the base of Edgar Pangborn's A Mirror for Observers (1954). Some aspects of the Philip Jose Farmer's World of Tiers universe also fit, notably the devilish Red Orc character, and a mainly-not-present benign creator.