I think the key words here are "never dip below 60 fps". Maintaining a smooth average of 60 is probably pretty easy with a single 1080. Dips happen for so many reasons, including the software itself, in the way that it is written. Every single aspect of the system, from a heavily overclocked multi-core, modern CPU, fast RAM, and fast storage would be necessary to minimize the chances, and even then, you might be chasing a ghost.
For The Division specifically, at 1080P with the highest settings available, I don't think you need more than a single 1070/1080, assuming the rest of the system's components are of the same relative performance tiers.
If SLI support is historically strong amongst the developers/publishers that you personally...