I would say it depends on your reading pace (which I know is a cop out). Reading both books combined is a pretty large task and keeping that much sprawl straight can be difficult if you aren't going through them quickly. If you are a slower reader like me, there are fewer POVs, smaller geography and you reach some resolutions faster if you go one at a time. That said, if you want to completely immerse yourself in that world for a while then reading both makes sense as well.
I have been through them twice and can say from my perspective - knowing what happened in ADWD absolutely helped when I reread AFFC. Possibly because I am not that close of a reader, but having the broader narrative already down really helped me appreciate the details the second time through.
I hadn't even considered reading them combined until reading this post, but I think for my third read (probably sometime after WoW comes out) I will try this out.