Why can't you got to Spain, stay there 1-2 days and then go to Poland on a single entry Schengen visa?
You will have an entry stamp in your passport from Spain so there's no issue there that you did not use the Spanish visa as intended. Visitng Poland is merely moving around the Schengen zone, which is allowed hence "Schengen visa".
There's no reason you can't enter to Spain and then get a fight to Poland. Once you are in the Schengen zone, your passport doesn't get stamped or checked again until you exit the Schengen zone. I don't understand the purpose of hiding plans to visit Poland, there's no "work" or "study" connected to visiting Poland for the interview, so hiding that seems weird to me. Is the concern that if you told them you also want to visit Poland that they would decline you and tell you to apply for a Polish visa?