The purpose of having a corresponding author is that the editor has only one point of contact with whom he/she communicates and does not have to correspond with multiple authors. Although you are the first author, ideally you should not write to the editor if you are not the corresponding author. Since the editor is already in communication with the corresponding author, your writing to him/her may just lead to confusion. The corresponding author can write to the editor again asking for a status update. there is nothing wrong with that. In fact, the corresponding author can follow up with the editor once every 3-4 weeks.