The effect of uncountered lift is to make the plane rise. Rising on it's side is the same effect as a simple inside turn. To counter this rising action, apply a small amount of down elevator. This should keep the plane tracking straight along in a perfect 90 degree bank. Now, if anything, I'm sure you at least appreciate the difficulty of a seemingly simple maneuver.
Trainers are even harder to knife edge because of the dihedral.
Some won't even do it. Once you can proficiently knife edge in both directions, you might try outside circles. First establish complete knife edge, then apply more down elevator. The plane will push around a circle. For a real challenge, try a knife edge circle with outside snaps at the three, six, nine, and twelve o'clock positions of the circle, stopping the snaps on knife edge and continuing the circle until the next snap is due.