Simultaneous log ins are not allowed. Also, it's probably better to do the Q banks on your own since then you can focus on what your weaknesses are. Unless you are going to do all the Qs together, I would not recommend sharing.
I have a similar question whether we can share the NBME subject exams. I figure that myself and my friend can each purchase an exam in tutor mode so we have a longer time for each of us to go through it, but once you submit the exam for grading, can you go back to the exam?
You won't be able to see how YOU do. or which type of Questions YOU miss. these two things are important to YOU. so to answer your question you can. you cant both log in at the same time, its against the rules(which some how isn't as important as the next point), and you don't get the best possible feedback.