The GeForce GTX 960 SLI is not just undone by its own shortcomings due to a lack of perfect scaling in some games, but in being a whole $70 costlier than a single GeForce GTX 970. The GTX 960 SLI ends up offering roughly the same average performance as a single GTX 970 across resolutions. You're, hence, much better off choosing a single GTX 970 to GTX 960 SLI; that is, if you plan on buying two of these cards outright. The GTX 970 offers close to 20 percent more performance per dollar than the GTX 960 SLI in 1080p and 1440p.
The narrow 128-bit memory interface per card and low 2 GB per GPU memory let the GTX 960 SLI down, particularly in higher resolutions. In a multi-GPU scenario, an app is dealing with dedicated memory the size of a single card (2 GB in this particular case)