It is well-established (and typically spelled-out in journals' instructions to authors) that is unacceptable to submit a single manuscript to multiple journals for peer review simultaneously. However, the rules for presubmission inquiries (which are essentially cover letters asking whether the journal would be interested in reviewing a given paper) are not clear to me; a presubmission inquiry does not constitute a formal submission.
Is it permissible to submit a presubmission inquiry regarding a single manuscript to more than one scientific journal?
Is it unethical to do so?
Further details on what presubmission inquiries are:
Presubmission inquiries are relatively common in genetics and biology. From the Nature website:
Presubmission enquiries are provided purely as a service to authors (for example, to see if a manuscript is likely to be of interest to the journal) and are not compulsory... Presubmission enquiries are not required for Articles or Letters, and can be difficult to assess reliably; Nature editors cannot make an absolute commitment to have a contribution refereed before seeing the entire paper. Entire papers should not be sent as presubmission enquiries, but should be formally submitted through our online submission system.