My daughter, who is a dual citizen (US and Spain), is returning from Spain after being there for 9 months studying. She entered Spain on her Spanish passport. She will check in for her flight back to the US with her US passport (proof to Iberia that she can enter the US) and enter the US with her US passport as required by US law. The trick is the exit from Spain as there is no entry stamp on her US passport, since she entered on her Spanish one. The suggestion seems to be that while she is flying on her US passport that she goes through EU passport control on her way home with her Spanish passport as there's no stamp in her US passport.
A potential issue could be that if the Spanish ask for her boarding pass at passport control when leaving Spain, her name on her US passport and thus her boarding pass is slightly different than on her Spanish passport. In the US, she has just my husband's last name and on the Spanish passport, she has his name and my name (the sedan family name). Thoughts?