I tend to think of SLERP as a rotation about a fixed axis of rotation, where the angle is linearly interpolated between 0 and the final angle.
It's not clear to me what you mean by the "same factor value", but I assume you mean that the angle between A and C is the same as the angle between B and D.
Without getting in to a mathematical proof, I would say that A and B only stay orthogonal if both of their SLERP's use the same axis of rotation. I'm just coming to this conclusion by example.
e.g., Example 1 -- all vectors in the same plane, A=(0,1,0), B=(1,0,0), C=(0,-1,0), D=(-1,0,0). Both A and B go through a 180 degree rotation. Since SLERP rotates them both at the same rate, at t=0.5 the will both be 90 degrees from where they started, and still be orthogonal.
Example 2 -- drastically different axes of rotation: A=(0,1,0), B=(1,0,0), C=(1,0,0), D=(0,0,1). In this case, A is rotating around the Z axis by 90 degrees and B is rotating around the Y axis by 90 degrees. Without taking the time to prove this mathematically, you can do a finger-proof to demonstrate that you can't keep the vectors orthogonal while they are SLERPING. just try to hold you thumb and forefinger stiffly at 90 degree angles (assume that the vectors remain orthogonal through the rotation), and you will see that as you SLERP vector A towards C, vector B cannot take the shortest path towards D. Since SLERP takes the shortest path, this is a proof by contradiction.
