The corresponding author takes responsibility for the manuscript during the submission, peer review and production process. All communication from submission to publication will be with the corresponding author.
The corresponding author does not need to be the first author or a senior author. The order of authors can be arranged during the submission process, in whichever order suits but submissions must be made by the corresponding author and not on behalf of the corresponding author.
Articles can be published with more than one corresponding author, but only one can be accommodated by the Peer Review System. For that, make sure that all corresponding authors are listed as such in your manuscript.
The logic behind only allowing a single author to access the manuscript is that only one author should take responsibility for the manuscript during the submission, peer review and production stages. Having a single point of contact ensures that the manuscript can progress rapidly through the process and that all communication from Springer Nature regarding a submission goes to a unique contact.