It's very, very rare that a game gets SLI or CFX support at a low level in the code. Devs are lazy and cheap, it costs money to code support.
It's usually left down to Nvidia and AMD to get the scaling right. Different engines and different code needs to be scaled differently and YMMV.
There are only two to three ways scaling can work, so if that doesn't work you've had it (see also every UBI game ever).
AMD are putting the ability to change it yourself back into the driver. There used to be a tool that allowed you to change profiles, now the drivers will be going back to the older style and allowing you to change it yourself in the hope it can get the profile working quicker. However, AMD state that sometimes they can add other optimisations that this driver tool won't let you change, offering up even more performance.
As an example here. If a game dev made a game for SLI only then it would not work properly on one card. As such they would need to sell different versions of the game. It's just not economical.
All is not lost however. New APIs like DX12 and Pascal allow GPUs to talk to one another at a low level so according to them you will be able to run as many as you like, different makes and even models and they'll all scale..
Nvidia offer you the profile changing option in their drivers now.
