I think dual-SIM presents a relatively limited business opportunity. I'm sure it's wonderful for the people who need it, but I don't encounter very many people who do need it. Building the cost of that second SIM slot into every iPhone sold doesn't seem like smart business, nor does producing a special, dual-SIM model. I could see it as a standard feature of the "Plus," just another reason to spend more on that model. :::shrug:::
I find many dual-phone users also need to segregate their data - conforming to corporate policies on the company phone, while wanting the freedom to do anything they please with their personal phone. Those companies that apply a profile to their devices would need a way of doing that without interfering with the personal usage. That might be addressed by a phone with multiple User accounts - perhaps choosing the phone line automatically switches the User account? Altogether, an interesting list of challenges.