More than one corresponding author for a paper?
Being corresponding author does not primarily concern the journal publishing process. The role of the corresponding author is to handle communications with other researchers once the paper has been published, to respond to questions concerning the research as a whole, methods, results. It is typically the most involved person who takes on this task and in many cases it is implicitly understood it is the first author. So for you to take on the corresponding author task means much more than the communication during the publishing process. 

I can see reasons for there to be more than one corresponding author, although I have never encountered it. This would be if the paper contains material where two (or more) authors are experts on separate parts and it is necessary to contact each for specifics on their specialty. As I said, I have never seen it and am not sure it has ever been done. Perhaps we might find out through comments to the answer...