I'm not sure I understand the issue here. If (per the link in Xero's post) the AA system will let you temporarily have a duplicate reservation for about an hour, and the original concern of the OP is that they don't want to cancel the currently held reservation until the new one is in place... then why not just go ahead and create the new hold under your own FF#, and a few minutes later (but definitely less than an hour) cancel the originally held reservation? What's the point of NOT putting in your FF# for the newly held reservation in this particular scenario, where the OP clearly wants to cancel the original hold immediately after the new one is in place?

I understand the general point of "one with FF#, one without FF#" if you want to maintain duplicate reservations for a long time, but that isn't the goal here.

I must be missing something!