Are you planning to include the PPCB UniCab or pending ROG Condor cab sim with one or both of the Preamp circuits mentioned in the OP?
The thread title "2 in 1 vs 2 pedals for cab sims? Opinions?" talks about cab sims, but the OP doesn't mention them apart from the Walrus reference.
Two preamps and a cab-sim (or two) will be a tight squeeze...
I would build both circuits and before boxing try stacking them; first the Cobbler > John and then John > Cobbler at a variety of settings low gain > high gain & viceversa etc.
Then I'd decide whether to include them in the same box or not, and whether or not to include an order toggle based on the unboxed experiments.
If you know you're going to keep them together it makes sense to put them in one box, saves some real-estate on your pedalboard. You can then also decide whether or not to have independent jacks for each — with a switched-jack setup you could have an FX loop to insert other FX between them, or just go for a simpler single in/out for both and save $ by using less components.
Pretty sure you'll need at least 1590XX sized box to house them both.
If they don't stack well, I'd build them as separate pedals for greater versatility — no sense in having a huge 1590XX on the pedalboard when you only ever use half of the circuits inside. Mind, if they don't stack well and yet you still love both circuits sound and want to switch between those sounds then a 2-in-1 with a footswitch to toggle between them makes sense again. Given the size of the enclosure required to house both effects, it gives plenty of room for two footswitches instead of a single toggle and single footswitch.