Recently, USCIS Service Center Operations offered an opinion on whether it’s possible to file a second visa petition if the first one was denied and is currently on appeal or a motion to reopen is pending:
Yes, a second petition can be filed. However, under longstanding Service policy, the second petition will be held in abeyance pending the outcome of appeal. If the petitioner would like quicker resolution of the second petition, the appeal should be withdrawn. Please see the 2-8-89 Richard Norton memo, entitled Adjudication of Petitions and Applications which are in Litigation or Pending Appeal which discusses this policy.
Source: AILA-USCIS SCOPS Q&A (July 30, 2008).
Update: USCIS now admits that the hold-in-abeyance policy was not mentioned in the Norton memo, but insists that the practice was adopted following the memo. USCIS states that the purpose of the practice is to ensure consistency in adjudications. See USCIS-AILA Liaison Committee Agenda (Oct. 28, 2008).