"Screen" tab means you are enlarging the D800 more than the D750. Select the "Print" tab and there is essentially no difference between them.
I am not enlarge anything.
Everything gets printed at some size. To put it in perspective, at 300 dpi you would be viewing the D750 file at 20x13 inches at say 10 inches viewing distance compared to a D800 file viewed at the same viewing distance that's 24x16 inches in size. Viewing on your monitor at 100% you are looking at the print equivalent of 5 feet across.
This is the real difference between these two sensors.
No, it's the difference between the pixels. It says a lot to me that you confuse pixels with sensors.
The print is a great equalizer that successfully masks the differences.
Put another way, printing or viewing on the web equalizes the differences between these cameras, which is why in practice there is no difference between them.
One thing you don't get is that resolution is also part of the equation. When there is more resolution the file can take more NR and still show the same amount of detail, and that's in addition to sizing both files to match output.
Here is ISO 6400 from DPR's studio comparison:
One is from the D750 at 100% and the other is from the D800 sized to match the D750's output. Anyone that gets worked up about the differences between these needs to get a life.