If you look at your 8PreX manual, you'll see a diagrammatic and instructions for exactly the set up you are trying to achieve.
From memory, I think you need to set up the two optical connectors exactly as you describe but you also need a word clock cable between the second octopre (ie the one you intend to be channels 17 to 24). Then you set the first octopre to clock master, the 8PreX to adat and the second octopre to word clock.
I think that's how the Focusrite manual says to do it.
Good luck and let me know how you get on ?
I find focusrite control the hard bit . . I tried exactly the same set up you are attempting some time ago so as to have 24 channel recording. . . . but . . . I'm just not good with software and just not clever enough to understand what to do with focusrite control . . I got all the hardware talking to each other fine I think as nothing flashing no errors or anything . . but I could only ever see channels 1 to 8 in focusrite control . . . I messed about with focusrite control (which I don't find intuitive as I'm rubbish with sorfware) for many many hours . . in fact days . . . but eventually gave up as the software was beyond me and I still only had 8 channels ? . . .
So in the end I’ve given up and will probably sell the octopres . . . . they were no good to me if they could only be used with software I couldn't understand !!
[ Post last edited on 09/03/2021 at 11:02:57 ]
As I say this bit is easy . . the last paragraph in the above instruction “routing using focusrite control” is an absolute nightmare and try as I might I could not do it !!!!
[ Post last edited on 09/03/2021 at 11:12:38 ]