Supposed you have two gfx cards it's impossible to specify which one should render which viewport. Load balancing is done by the driver in the background.

I'm not a OpenGL programmer but I'm a DirectX one. . . What if you enumerate the video devices and create a virtual device for each physical video device found? Wouldn't it work?!(at least in DirectX I think it would do) . ..

As said, if they're on the same screen they'll probably go to the same card.
Just create the window in the correct screen to select the proper card (supposing the driver lets you do that - performance is often screwed on secondary adapters).
Enumerating how the screens are connected to 3d cards takes a little of care. I believe the D3D approach to be the easiest (checking DEVCAPS for the adapter ordinal and head index).

Load balancing typically NEVER works across different 3D cards. The load balancing as implemented in SLI setups work on a way different method in which the SLI cards are collapsed in a single multi-multihead card. They are then handled as a single device in d3d terms.

If you have multiple D3D devices (or multiple GL render contexts) there's no way balancing can happen. The driver may trick the system in some way for better performance but there's no chance it will ask another card to "help". There would be no way for the two cards to comunicate and transferring GBs of framebuffers across the PCIe is never going to work...