Assuming that your SLI is working in general (check in Nvidia control panel > Configure SLI), with recently downloaded Nvidia profile inspector, I tried the following:
This was inspired by what worked with earlier M&B. The only issue is so light flickering, also a problem with earlier M&B (which tells me we are really dealing with the same engine), not much a problem by comparaison to performance benefits.
Checking your system performances, afterwards you should see both GPU working (here, just after loading a savegame).
There is probably room for fine tuning but that's already a start.