I've seen these words used in the following sentences:
以後: それ以後彼女に会っていません 以降: それ以降彼は辛い一生を送った
Judging from these examples alone, I guess they have some kind of overlap.
When is it appropriate to use one over the other?
I tend to think of
以後 as meaning
one or more specific points after whereas
most of/the whole time after. But I'm not positive that they don't overlap at all.