Save those for the couple of weeks before your boards. They are a great way to assess your readiness. They are one of the only resources that gives you a score you can actually correlate to the real thing.
I only did one. I couldnt justify another $45 to do the second one.
I would definitely support buying both. As previous poster stated, they are really the only tests that correlate to the score. By averaging my score on both and using a comparative mean and SD, they accurately predicted my Step I score. I took them in the last week before the actual test.
Last year you had to buy them separately, $45 for 20 days of access to each one. I bought them within days of each other. Remember, they do not provide answers or explanations, only an assessment of performance in the different categories tested. So if you are intent on learning from them, that is not really their purpose.