There is no orderly generic way of making it work, nor a reference for deterministic diagnosis of what might be missing in a particular configuration, at least none supplied in any of the answers or available online. In that latter regard, it is utterly disappointing.
Maybe one day upgrading from 14.04 to a newer release, or newer Nvidia drivers would make it possible, but for now there's a myriad of layers of software and software settings involved, which provide little to no proper and relevant diagnostic information.
It might be the case that with Nvidia cards lacking SLI support this cannot work, but the current state of documentation leaves that at the state of a conjecture.