1. If money was no object what would be the best configuration to order for a new Mac Pro to handle Lightroom Classic most effectively, given that clock speed goes down with the number of cores on the machine?
The 8 cores of the base model are already more than most Mac applications, including Lightroom or Photoshop, can use effectively most of the time. Cores above 8 are of most use to video editors or maybe scientists.
Facial recognition on my large collection of 130,000 photos is currently painfully slow!
I'm not sure this can be solved with money. Significantly speeding up facial recognition may require additional optimization by Adobe, like using a more efficient algorithm or finding a way to do it on the GPU .
2. The machine can be configured with 2 SSD's
Would you put all Lightroom files on one SSD; or split them with Previews on one SSD, catalog and and photo files on the other SSD? (To minimize file access times)
The flash storage is the fast NVMe type used in other Macs, several times faster than common SATA SSDs. As clee01l said, the images don't have to be on the fastest available drive because after an image is first loaded into Lightroom Classic, what matters is the speed of the drive where its data is cached for editing (the Camera Raw cache location set in Preferences, & the Previews location).
The flash storage is so fast that for photography, you might not notice any practical speed gain from splitting data across multiple SSDs, although it probably would be a little faster if you measured it. The people who would notice are video editors working intensively on 4K or higher media. Also, with flash storage, another way to speed it up is to simply buy a bigger drive, since larger capacities tend to have more data channels running in parallel to the controller. But again, for photography specifically, you'd hardly notice the difference since the drive is already so much faster than is needed for Lightroom.
3. I'm assuming maximum memory and 2 GPU's make sense, and will speed up file handling. Correct?
No.
On Mac desktops, the days where you should always "max out" your RAM are disappearing. If you edit single images (as opposed to merging to HDR or panoramas) with Lightroom or Photoshop, most of the time they won't even need 32GB RAM, probably between 16–24GB. But all desktop Macs can now take 64GB RAM, the iMac Pro can take 256GB of RAM for $5,200, and the new Mac Pro can take 1.5TB of RAM for $25,000. So you could "max out" the RAM on that Mac Pro (instead of buying a new car or kitchen with the same $25,000), and find out that most Mac applications including Lightroom are only able to use a tiny fraction of it.
2 GPUs do nothing for Adobe photography applications. The Photoshop tech notes specifically say that it cannot use multiple GPUs.
In general, the GPU story is that the Lightroom and Photoshop teams are progressively adding more GPU optimizations to those applications over time, but most operations are not GPU-accelerated. For Lightroom and Photoshop, the main factor affecting performance, including Lightroom exports and Photoshop saving, is still the speed of the first 4 to 6 CPUs cores. So the base model GPU on the new Mac Pro is probably fine. The lowest GPU upgrade ($2400) might be considered if multiple very high resolution displays (meaning 5K or above) will be connected.
The answer to the first question is that if money is no object, you still would limit the upgrade options to the few that Lightroom could take advantage of. You could pour more money into it, but that wouldn't improve performance much for Lightroom. The activity that really takes the most advantage of the new Mac Pro upgrade options is very high resolution video editing (and probably 3D, VR, game development...). For most other creative activities including photography, the base configuration of the new Mac Pro may already be more than the applications can use, with the main justification for buying it being that it's the only modular Mac desktop option other than the Mac mini.