I see Vray has just announced this in their 3.5 update. I remember Maxwell announced it about 10+ years ago but did they ever get it to work? I know in V1 when I bought it, it didnt work. Funny that light mixing is only coming round in other renderers now as multilight is pretty old news as a feature. I'm looking mainly at vray here, as Corona was relatively quick to implement it.
Anyway the resumable rendering feature is so appealing to me with long render times and sleepless nights praying nothing will interrupt an over night rendering under the heat of a deadline. Having the option to save out periodically as Corona renders would be a great intermediate solution (I know this will sacrifice a bit of speed) but then the full blown resumable render feature would be fantastic and also opens up the possibility of moving the render from one machine to another.
Cheers