The Division does seem to get decent SLI scaling, so you might be OK with a second 980 Ti, but the 1080 Ti is for sure better overall (especially considering games that don't support multi-GPU).
SLI is on the way out, unfortunately, from what I've seen. At first DX12 discrete multi-gpu sounded like a great thing, but so far at least it seems to translate to "more work for developers, they don't want to bother, single GPU only from now on." Vulkan's multi-gpu support just recently announced will probably end up going a similar route. Fastest single card is king yet again... It's just a shame none of them can manage 4K at 60fps with maxed settings in most titles.