My NSK paper only showed the df/dt only, as that's what's used in my MAHO and brother machines. I guess they will make them any way anyone wants...
From SKF:
"If misalignment cannot be avoided between the bearing positions, face-to-face bearing arrangements are recommended. They are less sensitive to misalignment than back-to-back bearing arrangements"
https://www.skf.com/ca/en/products/b...nts/index.html
THK BKxx, the most common low cost support style for the type of machines built on this forum:
My Brother tapping centres use face to face (NSK 20TAC47BDF) bearings
The MAHO bearings are nsk TAC25 and TAC30 face to face
Here's the face to face / tandem setup in a mori lathe:
Attachment 434974
There are a handful of examples of back to back in machines I've found in a quick search, but almost everything is face to face, especially when just using 2 bearings (you would lose some of the alignment benefits with more than 2 bearings or when spacing out the bearings).
On the very bottom end of cheap I should add that sieg uses 2 thrust bearings with a double lock nut and Belleville spring, on a 12mm c5 ground screw, and honestly, it worked better than the angular contact bearings in some other machines I've had because you can adjust the preload precisely.
So, to the OP... look at what other machine makers do, and what the bearing manufacturers say, and don't blindly listen to what someone on a forum tells you - they *might* not have a clue what they are talking about it turns out.