Yeah this question doesn’t really make any sense. For one, I have yet to meet anyone who has pursued an MD/PhD specifically to get a PhD in physics. The purpose of the MD/PhD is to train physician scientists to perform, ultimately, translational research to improve the health of the public (as per NIH). I suppose you could get a PhD in medical physics..though even this is rare.
Also, as stated above, a second college major is just coursework. Doing research in a lab relevant to the PhD topic is more likely to be helpful to you.