Not sure you will understand this.
Flash drive is just like normal harddisk without the moving parts. For every harddisk, there is a seek time, read time and write time. If the size of the drive is big, seek time will be longer and it would take longer to read a file as compare to write.
However, comparing 16GB and 8 GB flash drive the timing is not that significant as compare to today harddisk of 80GB and 250 GB. So the amount of time in reading 16GB flash drive is and will not appearing at all. You might not find any difference in some or many cases.
The risk to take for 1 x 16 GB compare to 2x 8GB is just plainly no backup disk to use if the 16GB flash drive is malfunction. That mean if your 16GB flash drive is dead, you will lost all data. if you have 2x8GB disk, you might just lost 50% of the data compare to 16GB. not sure you will get what I mean.