Can there be two corresponding authors?

No. Currently JMIR Publications supports the display of only one corresponding author.

However, we can generate a footnote saying that all or a certain number of authors (e.g. first and last author) contributed equally to the work. To generate the footnote, click the checkbox "equal contribution" in the metadata form for each author which should carry this footnote.

In addition, authors can use an "Authors Contribution" section within the manuscript (after the "Acknowledgments"), see What is the "Authors' Contributions" section? How should I write mine?

Two different groups of equally contributing authors

While we are able to provide equal contribution footnotes on manuscript, it is only applied to one equal contribution. If Author A and Author B contributed equally, and Author C and Author D also contributed equally (but differently from Authors A and B), we would recommend to include this information in the Authors' Contributions section of the manuscript. Alternatively, one set of equal contribution (Authors A and B) could be listed in the author list footnotes, and the other equal contribution (Authors C and D), could be listed in the Authors' Contributions section.