The lengthening of the refereeing process may encourage authors to submit the paper to another journal, which put their reputation in danger.
If your field of study or the results of the work are very specific and specialized, it is very likely that both journals refer your article to the same referee. The referee can inform the editor-in-chief and put the authors’ names permanently in the blacklist of the journal (s).
Most of the journals ask the authors to send a cover letter that commits the article has not already been formally published or has not been submitted to another journal.
There are two reasons for the prohibition.
The main reason is the referee’s wasting time, which is so valuable for the editor. One of the most difficult responsibilities of the editor is to find appropriate referees and convince them to accept and spend time to peer review.
The second reason is to prevent the publication of a previously published article or simultaneous publication of an article in several journals.