According to NVIDIA's Andrew Burnes, the Just Cause 3 rendering engine is not compatible with both SLI and Crossfire, meaning that Avalanche will need to release a game patch in order to enable multi-GPU support.
The engine is incompatible with all multi-GPU solutions, so no SLI support at this time.
Sounds familiar ? Another gameworks title that really doesn't offer all that users expect. This is rather disappointing news and a huge miss from the game developer I must say.