Two offers! Congratulations! Having more than one offer gives you power. You may not be used to it. When you have power, use it gently. Because in this case, it's the power to disappoint the ones trying to hire you.
You don't burn bridges if you respectfully decline any offer of employment. Just say, "Thanks for the wonderful opportunity; I really wish I could work with you but I just accepted something else." You don't owe them an explanation of why.
If they say "keep in touch," respond, "I intend to." Then do keep in touch. At a minimum send a hand-written note thanking them for the interview and the offer. If you have another chance at an internship later and you're still interested, ask them again.
If they ask for an explanation -- "what company are you going to?" etc -- you should listen carefully to their question, then decide whether you want to answer it. If I were a hiring manager, I would say "Congratulations. That's great for you if not for us. I'm curious, where are you going to work?"
It's generally disrespecful to you if the hiring manager pressures you to change your mind. If that happens, you'll be in a tense situation. You can always say, "thanks for your interest in me, but I have made up my mind." Remember, you have some power in the situation.
You DO burn bridges if you walk off one job or internship to take another. I know you're not planning to do that. Still, don't., OK?
You want to leave your second-place hiring manager thinking of you as the "big fish that got away."