It really depends. A big company can afford to have specialised Android devs etc, while a startup might want to shift "the mobile developer" between OSs and may

It really depends. A big company can afford to have specialised Android devs etc, while a startup might want to shift "the mobile developer" between OSs and maybe the web too. Another way to look at this question is to ask under what circumstances would you expect a developer on X platform to shift to Y platform. IMO it takes 2-3 months at least for a good developer to become professional-level in another language. And in an app context, it's not just a language, but a large set of libraries and frameworks. So you can assume a 4-6-month ramp-up in skills, with relatively low productivity, and then work backwards to decide if the investment is worthwhile. If the developer has sufficient domain and institutional knowledge, and is likely to stick around, it might be.