I am currently doing my Master in physics in the University X. Previously, I did my Bachelor in the University Y. I am applying for a PhD positions that requires two recommendation letters. The supervisor of my Master Thesis agreed to write for me a letter.

For the second one, I could ask a professor with which I did a course and a seminar in my current university X, or I could ask the supervisor of my Bachelor thesis when I was in Y. Who do you think is better? I add some information: I finished my Bachelor in physics three years ago (I hope he remembers me). I worked around six months for the Bachelor thesis and at the end it was awarded the highest grade. So in principle I would ask him, but I do not know if it is okay for PhD applications to have letters from the Bachelor.