Reneging: The Office of Career Planning & Professionalism will not support a student who reneges on a commitment to an employer, except in extreme cases of unforeseen circumstances or of personal emergency. This means that a student who reneges without cause may not use any CP&P services or programs. The offense may become a permanent part of the student's law school record. Continuing to conduct a job search and/or interview after having accepted a position is considered unacceptable as well. A student should accept an employer's offer of employment only when he/she is committed to working for that employer.
Many unpaid internships will allow you to split.
I agree to check into splitting. But even if B is your dream job, I really do think you need to stick with A if splitting is not a possibility. If splitting isn't a possibility, I'd explain to B that you are really interested in the opportunity and would love to pursue it next year (if that's the truth) but unfortunately you've already committed to another employer for this summer.