Biaxial bending is bending about each principal axes of a members cross section, this is quite different to torsion which is a twisting along a members axis.
Biaxial bending does not lead to torsion directly. Though a sections response if it begins to fail in a flexural torsional buckling mode due to flexure about the major axis does mobilise a sections torsional strength and stiffness to resist the resulting twist as it becomes increasingly unstable. So in a sense some torsion is being generated by the instability of the critical flange as it wants to displace laterally.
Are you talking about flexural torsional buckling, or axial torsional buckling. Neither are caused by applied torsion. It is more the response of the cross section under either bending in the case of flexural torsional buckling, or axial load causing twisting along the section for axial torsional buckling (for example cruciform sections are susceptible to this type of failure under axial loading).
You need a bit of a more detailed explanation of your particular concern to get better answers, you question is a little high level and you'll likely get all sorts of answers as people try to guess what your concern is.
