So, many thanks and credit to @Erithon for connecting me with a Subterranea again! The stars aligned and I also happened to snag a Dubterranea at about the same time from Reverb that matches almost exactly, which makes the pedal nerd in my quite happy.
After getting a chance to compare them side-by-side, here are some brief thoughts about the comparison:
Tracking
As other people in this thread have reported, the SubT does track noticeably better. I was kind of expecting the worst with the DubT based on people's descriptions, but it's actually quite usable with good technique. Clean note attack and maybe back off the treble/tone control just a bit in some cases. Taylor has some other notes in the manual as well. After years of using the Future Impact and C4 which also demand the same thing, I didn't have much trouble. If I had to put a description on it, the DubT feels only slightly more glitchy than an OC2. But as a disclaimer: I didn't do a direct comparison with either of the OC2s I currently own.
EDIT: I also admit now that I create this post and look at the photo, I did have the SubT plugged into the DubT, so maybe the DubT would track a little better if it were first, with a direct signal? Or, I do have a compressor at the beginning of my pedalboard signal chain with light comp settings, which I find helps with tracking on my synth pedals. That may make a difference on DubT tracking as well, but I'd have to try to be sure.
Sound
There's a definite difference here that could justify having both on the same board, if you're looking for different sounds/use cases. The Sub very much rides on the more synthy/bitey side, while the DubT matches the sub sound of the OC2 quite a bit more closely.
And it might cause some waves, but... I actually prefer the sound of the Dubterranea.
*crowd gasps* lol
I also noticed that there seems to be a slight shift in the mids of the clean sound when the SubT is on. As in, without any sub octave sound. It's not a big issue because the purpose of these pedals isn't to have just the clean sound, right? But I still thought it was worth noting. The DubT doesn't have that shift in the mids.
The filter on the Sub isn't as "OC2-y" as the Dub, but has a wider sweep. It really starts kicking in about 1:00 with a lot more high frequency available between 1:00 and 5:00. The DubT filter has much more LOW frequency content, in the "air movement" territory (think Meatbox), and the Timbre strikes me as very similar to the Octabvre MkII tone control.
Settings comparison
Unity volume for clean on the Subterranea was right at 12:00, versus 2:30 on the Dubterranea. The volume of the sub controls seemed to be the same on each respective pedal, but that's always harder to get specific about with those low frequencies.
5sg.