Two pbuffers and NV VAR2 poor performance

I’m using GL_VERTEX_ARRAY_RANGE_WITHOUT_FLUSH_NV (VAR2) to render my whole scene and I found a problem. I’m rendering on a pbuffer called main_pbuffer. There is no problem if I use only one pbuffer, but I want to make depth shadows, so I need to use another pbuffer to render the scene.
Here comes the performance problem:
To get good perfrmance I need to call
glVertexArrayRangeNV in active pbuffer (main_pbuffer) like this:

glVertexArrayRangeNV(mem_size*sizeof(GLfloat), mem_buffer);

If I call glVertexArrayRangeNV outside active pbuffer I got very poor performance.
And now I need to render whole scene using the same vertexarray (cause it’s impossible to create another) on another pbuffer. And on that second pbuffer I got very poor performance, because glVertexArrayRangeNV was called only on the first pbuffer.

Does someone can give me some advice?