Scratch drive is a phrase I've only encountered in that other thread so your confusion is justified!
To say that one drive writes and the other reads is a bit misleading...what's the point of writing to a drive if it doesn't read! (And vice versa of course!)
Anyway, my interpretation (based on the linked article and the way I operate too) is that it's good to have one drive that contains your operating system and DAW software (so, other than internal housekeeping, it's only reading. A second drive is there to contain the actual audio files you're recording and working one. This allows both drives to work at the most efficient. So basically you're doing it exactly right!
Why some people call them scratch drives I don't know!