It depends on the dye method. With the Fabric Dye method, you want to dip half your wig into the dye bath and keep the other half outside of the bath. You may want to use clips to keep the separation. With the ink-dye method, you’ll want to part the wig and mask off the part that isn’t getting dyed. I would put the one half in a lastic bag and use masking tape along the part.
However, wig splicing is a lot cleaner and easier to get the result you want as long as you have the budget for two wigs. Pretty much you cut two wigs (carefully) down the middle and sew the two sides together. This is a great visual tutorial to walk you through it.