Do you even have a motherboard that supports SLI? For the i5 7400 only certain Z170 and Z270 boards have SLI support. As said above, SLI isn't really worth it anymore, very few games support it these days. Even if game support was better, the i5 7400 is too slow to really benefit outside of maybe 4K resolution or higher. On the older LGA 1151 platform only the i7 6700k and 7700k would come close to not being a bottleneck, and that's assuming you don't hit PCI-E bandwidth limitations, some people who have tried SLI 1080Tis on mainstream Intel problems have run into that problem as an x8 x8 configuration apparently doesn't provide enough bandwidth, meaning you'd have to look at something like X299 or X399 to get around that problem.