I\'m developing an OpenGL application for Windows XP. The target machine has 2 NVIDIA GeForce 9800GT video cards, which are needed because the application needs to have output 2
I beleive you can gain such information from the gDEBugger for OpenGL based applications.
If it turns out your not using both cards, you can check out Equalizer for parallel rendering, it's a great project.
I think you need to read up on the WGL_nv_gpu_affinity extension. You create affinity masks and use wglMakeCurrent() in conjunction with them. Here are some pointers:
http://developer.download.nvidia.com/opengl/specs/WGL_nv_gpu_affinity.txt
Pdf from NVidia.com
Cheers !