Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor. On the other hand, Canon 6D MII has a 26.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features DIGIC 7 processor.
Below you can see the D750 and 6D MII sensor size comparison.
Nikon D750 and Canon 6D MII have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon D750 has 7% larger pixel area compared to Canon 6D MII, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.