Hi! I am a current senior looking to go into the medical field. I can tell you that it is definitely possible to take 2 AP sciences together. I personally took both AP bio and AP Physics my junior year along with other AP Courses and am currently taking AP Chem as a senior. I can tell you that many students struggle with AP Chem and AP Physics because there is a very heavy emphasis on understanding the concepts and applying it to various math based problems. AP Bio will be challenging if she struggles with memorization and application. That being said, extra time put into the course work and proper time management will be essential! As for specifically what aspiring med students should take - I would recommend taking all the AP Science Courses as that shows well rounded exposure to the field.
While course load is important, make sure she participates in medical related activities such as volunteering, shadowing, research, etc. if she wants to go into the medical field. And more important, it is important that all of the activities are concentrated throughout the course of high school and not just in 1 summer.
Good luck!