This shot through my brain like a chain of firecrackers, so, without derailing the original post, I have some THOUGHTS to add about why this concept is not only hilarious (because it is), but also...
It. It kind of fucks. Severely.
And in a delightfully Pratchett-y way, I'd dare to suggest.
As inferred above, both Crowley AND Aziraphale have canonical Biblical counterparts. Not by name, no, but by function.
Crowley, of course, is the serpent of Eden.
(note on the serpent of Eden: In Genesis 3:1-15, at least, the serpent is not identified as anything other than a serpent, albeit one that can talk. Later, it will be variously interpreted as a traitorous agent of Hell, as a demon, as a guise of Satan himself, etc. In Good Omens --as a slinky ginger who walks funny)
Lesser known, at least so far as I can tell, is the flaming sword. It, too, appears in Genesis 3, in the very last line:
"So he drove out the man; and placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life."
--Genesis 3:24, KJV
Thanks to translation ambiguity, there is some debate concerning the nature of the flaming sword --is it a divine weapon given unto one of the Cherubim (if so, why only one)? Or is it an independent entity, which takes the form of a sword (as other angelic beings take the form of wheels and such)? For our purposes, I don't think the distinction matters. The guard at the gate of Eden, whether an angel wielding the sword or an angel who IS the sword, is Aziraphale.
(note on the flaming sword: in some traditions --Eastern Orthodox, for example-- it is held that upon Christ's death and resurrection, the flaming sword gave up it's post and vanished from Eden for good. By these sensibilities, the removal of the sword signifies the redemption and salvation of man.
...Put a pin in that. We're coming back to it.)
So, we have our pair. The Serpent and the Sword, introduced at the beginning and the end (ha) of the very same chapter of Genesis.
But here's the important bit, the bit that's not immediately obvious, the bit that nonetheless encapsulates one of the central themes, if not THE central theme, of Good Omens:
The Sword was never intended to guard Eden while Adam and Eve were still in it.
The Sword's function was never to protect them. It doesn't even appear until after they've already fallen. No... it was to usher Adam and Eve from the garden, and then keep them out. It was a threat. It was a punishment.
The flaming sword was given to be used against them.
So. We have our pair. The Serpent and the Sword: the inception and the consequence of original sin, personified. They are the one-two punch that launches mankind from paradise, after Hell leads it to destruction and Heaven condemns it for being destroyed. Which is to say that despite being, supposedly, hereditary enemies on two different sides of a celestial cold war, they are actually unified by one purpose, one pivotal role to play in the Divine Plan: completely fucking humanity over.
That's how it's supposed to go. It is written.
...But, in Good Omens, they're not just the Serpent and the Sword.
They're Crowley and Aziraphale.
(author begins to go insane from emotion under the cut)