And after a whole year forgetting about them I may now continue :D
The order I follow is chronographical, just because it's an easily avaliable order.
But it's one of those series in which it does not matter at all.
I never really cared, until I realized I had no idea which ones I had read and which ones not. (But a selected few beloved ones.)
For example, those two books you mention I would say they are completely unrelated - other than the fact that they belong in the same fictional world.
For that kind of doubts I usually check the series on GoodReads:
https://www.goodreads.com/series/40650-discworld
As you can see, all the books are listed there in order, but also you can check some "subseries" which are actually books with the same main character(s).