I opened a topic about SLI a few months back. Didn't get a lot of info so I started researching myself. What I've found is basically this:
1) Yes, a game developer specifically needs to program for this, there are only a handful of titles that support it.
2) SLI is going to be phased out, just as 'crossfire', the AMD protocol. They're superseded by DirectX12. SLI and crossfire only work up till DX11. DX12 was made with multi GPU in mind, in fact DX12 does what SLI/crossfire did, it handles multi GPU's. So that's supposed to make things easier for game developers, since they're now only need to code against DX12 instead of several different protocols. However the problems remains: the game developer actually has to program specifically for multi GPU. Read more about DX12 and multi GPU here or simply google it to get more info
This pretty much sums it up though I guess: " I wouldn't bank on this particular feature becoming widespread quickly, it's lots of extra work for the game developer, and most developers probably won't put the time and resources into a feature that only a small subset of the PC audience uses. Ashes and Hitman only really have the feature because AMD threw money at the developers to showcase DirectX 12."
I really think we have to wait for NVIDIA to release Volta. It's too bad that AMD sucks so hard right now at making GPU's. NVIDIA doesn't feel any real competition and that's why they're not in a hurry to release Volta.