Granted, been a long time since I've taken the CCNA, but I took it in the 2 parts. I did however take the CCNP routing and switching tests combined. I think there is good and bad to both. You get the same amount of matierial in both tests, difference can be (just as an example) you might have 75 questions in each part, or the combined could be 85 questions. I kind of liked the idea of potentially having less questions on topics I didn't feel quite as comfortable with, however they might be weighted more since there are less questions that might get asked on a particular topic. Either way, it's a lot of material that is available to be thrown at you in a single test.
If I had to do it over today, I'm not certain which way I would go again.