Having two hives gives you an immediate comparison, especially as a beginner. If something doesn't look right, you have another hive to compare it to. While no two hives will perform exactly the same, they at least will show similarities.
If one hive performs really bad, and you don't have another to compare it to, you will accept this as normal until you get more experience.
Look around on this site, and see how often someone thinks they are queenless. It doesn't happen to every hive, but when it does, (and I think it happens more often to those of us who are still learning) the first response is to throw a frame of eggs and brood into the hive in question. Unless you have a generous local bee mentor, the best place to pull that frame is from another hive of your own. Granted, it is a little more expensive to set up two hives from scratch, but in my mind, for the insurance and the learning curve, that second hive is worth every penny.
Two hives opens up all kinds of options for resolving problems. What happens when you have a failing hive that is queenless going into winter? You let the die? Or you combine them with a healthy hive. As mentioned, when you think a hive is queenless you can add a frame of eggs and let them raise a queen, or not if they already have one. If a hive is recovering from something you have resources in the other hive to help. Just a spare queen and fresh brood alone is worth keeping a second hive. Then you have, as mentioned, something to compare to. If both hives are behaving in a similar manner you have more reassurance of it being normal.
good points. I may look into putting up two hives then...
I absolutely agree with the 2 hives... I started on only 1, just to experiment and to get lessons learned from it. Even though I haven't been through a winter I wished that I had another hive to make up the difference in the event of problems.