Good question. It only makes sense if the disease is contagious before the symptoms show. This would be disastrous.
It would certainly blow their “tried and true” theory out of the water......there are other possibilities:
1) she carried the contamination on her body, clothes, purse etc. home to him basically exposing both of them at the same time
2) they contracted the virus together from another source other than Duncan
3) the CDC us wrong about being contagious only upon the onset of symptoms