Standing "straight and tall" implies a pride in the way you present yourself, that you want to be noticed. If a person stood up straight, I would imagine that he/she thought about their posture. If a person stoop up tall, I would imagine he/she wanted to be noticed. If a person stands up straight and tall, I would get a picture of someone who is confident - a picture that I would only get partially with the two separately. It's a cultural issue rather than a pure grammatical one.

Actually, there is not really an implied difference. We sometimes use pairs of related words to express something specific, much in the way FDMaxey has explained. Other examples include "safe and sound", "peace and quiet" and "hale and hearty".

Thank you GGM. Your suggestions are very helpful to me. I found another way to explain what the phrase "stand up straight" symbolizes. That's "adherence to the principle." So that you have your principles and stick to them, and then you are proud of yourself, which is expressed as "stand up tall." Do you think that's OK?

Though this is a question from a while ago, I wanted to answer it, strictly from my point of view as a teacher, and as a previous dancer and gymnast. To me, standing up straight refers to the posture of the back. Standing up tall refers to my attitude and self-esteem, that encourages me to show my self-confidence, and "hold my head high".