Setting Scratch Disk to Same Hard Drive as Operating

Partitioning does not do anything for SSDs. That's just not how this stuff works. Linear memory addresses most of the time do not represent the physical configuration like the linear tracks on a platter drive would. It's much more important to have a large drive and always a minimum of free storage to allow the SSD's auto-repair and layout optimization functions to work and prevent excessive wear from too many write operations in the same areas, even if today's SSDs are a lot more robust than they were a few years ago.

Mylenium

I agree with D_Fosse.

One reason we used to partition hard drives is so the scratch partition could be quickly defragmented (by reformatting) without affecting the system volume. But even then it was an inferior solution to using multiple hard drives, because having two partitions in constant use on one hard drive meant the read/write heads kept having to waste time scooting back and forth between the partitions, slowing things down. Multiple hard drives worked better because each could concentrate on one thing, either system or Photoshop scratch.

With the speed of SSDs there is no (or very little) penalty for read/write at very different locations on the drive. And you don't defragment SSDs. With the speed of NVMe SSDs, it might be OK to have one big volume that is both system and scratch, as long as it's big enough for the scratch files for the image sizes you edit. If you do this you want the biggest capacity you can afford, because typically, the larger the SSD the more channels it has to the controller, for more parallel transfers, reducing the possibility of bottlenecks. Personally I aim for a capacity that can always leave 150–250GB free on my system SSD at all times, to leave room for the scratch/temp/cache files of various applications and the OS. 

If they'll be SATA SSDs, it becomes a better idea to put scratch on a separate SSD since they're so much slower than NVMe and closer to hard drive speed. Also, if this is not just about Photoshop but also another throughput-intensive activity, like high end video editing of 4k+ resolution media, then putting cache/scratch and/or media on a separate SSD can still be a good idea, no matter how fast it is.

D_Fosse advised not partitioning except for housekeeping. I agree but if you decide you have to divide an SSD for some reason, don't use partitions. I know we're talking about Windows, but on macOS it is now possible to use "containers" that are like partitions but far more flexible and easier to add/delete/auto-resize without destroying data; I have heard that Windows 10 also has a similarly new way of dividing up volumes that is not as limiting as partitions are.

Put everything on a large system drive. With SSDs, that's the most efficient setup and the best performance. Don't partition, it does nothing for performance, if anything the opposite. It would only be for housekeeping purposes.

The separate scratch drive is a thing of the past. It applied to spinning hard drives with a read/write head that could only be in one place at a time.

EDIT: seemingly conflicting advice here, but I stand by what I wrote. This is also in line with the official recommendations from Adobe.

Partitions would work. I would be cautious about putting all my eggs in one basket though.