Hi clawwd:
Since no one else has yet asked, I will. Why don't you post pictures of your WHOLE plant, and then present the difficulty which you are encountering? I will always hesitate to offer advice when I only have a small part of the story.
While I don't disagree with stone's experience and take on the Asian species, clawwd hasn't even provided the identity of the Wisteria at hand. There are many species and selections in this group of vines - native and exotic - though they are all relatively rampant growers under normal soils and moisture regimes in the eastern US.
You have mentioned that this is a "tree". That could mean that this is a top-graft of a named selection on a seedling understock, or it could be a rooted cutting (or seedling) trained up as a single stem. Thus, the need for more imagery to know more about what is being evaluated. What you are calling a split might simply be the space between two roots on a shallowly planted rootstock, or possibly something more worrisome.
Wisteria in general are so vigorous that I really don't think that gap in the stem/roots will last much past next spring's growth spurt. I think you have longer term considerations to decide upon - or not.
