I've given the exam in Oak Ridge [consistently serving as the second largest exam site] for the last several years. We generally get 20 to 25 candidates for Part I, and 12 to 18 for Part II, with 6-8 taking both parts.
I'm in favor of taking both parts your first year. Even if you fail, it's good practice for next year. I check the results each year and it seems that one or two people [nation wide] seem to pass both parts each year. I think that it's mainly luck. Some individual tests are well suited to some individuals. The main problem with taking both during the same day is due to mental endurance. After three hours in the morning, your mind slows down just enough that it's hard to grind through the six hour exam in the afternoon. A few years ago, they changed Part II from five to six hours. This seemed to help a lot more people finish, with time to check their answers, but now they seem to have made the exams more time consuming [kind of a Catch 22], so now you just have an hour more material to deal with.
I took the NRRPT the year before I took Part I, and found them to be very similar, having done no extra preparation for Part I. Therefore, any NRRPT study materials (e.g., books, software, practice test questions) would also be useful for taking Part I.
For Part II, I was a big fan of Jim Turner's books "Atoms, Radiation, and Radiation Protection" and "Problems and Solutions in Radiation Protection." I found that while Bevelacqua's book was more comprehensive, it was also more confusing. Stick with knowing the meat and potatoes thoroughly, and you'll get through just fine. Also, the Gnoll [Knoll?] book on instrumentation is outstanding.
MGM