how to split agp memory

I try to use AGP memory for a Video I/O System. Therefore I need two memory chunks. When I use wglAllocateMemoryNV I can only create one AGP memory chunk. How do I split these chunk in two parts and how do I get the second pointer (just add the needed picture size to the fist pointer)?

To get the second pointer, simply add the size of the first memory block to the pointer.