What would happen if you combined the world's deadliest diseases & viruses?
Viruses in the same cell can recombine (although they don't always do). This can happen by crossing-over of the genomic strands or reassortment of genomic segments. Therefore, you can get a new virus out of the fist two. The new one is not necessarily deadlier than the first two. If there's no recombination, the test subject will enjoy both diseases until the deadliest one kills him.