It is considered unethical to submit the same manuscript to more than one journal at the same time. If the authors want to withdraw the manuscript during its processing, an email has to be sent notifying them as to their intention, to the effect that the manuscript withdrawal form will be emailed for the signatures of all the authors. This needs to be completed, scanned, and sent back. A final letter from the editorial office for the withdrawal of the manuscript will then be dispatched. Electronic signatures will not be accepted.
Please note that without a formal letter of withdrawal, a manuscript is not considered withdrawn and the use of such a manuscript elsewhere will be construed as ethical misconduct.