If you start with 2, you have one hive you can compare with the other and if something goes wrong with one you may realize it soon enough to save it. If a queen dies in hive 1 and you realize it, you can rob a frame of eggs from hive 2, put it in and they probably will raise a new queen from it. A few factors on this, old queen from hive 1 must be dead and gone (bees will usually be meaner than normal) and they must not have supercession cells built from her last eggs or even worse going into a laying worker stage. There's also the emotional standpoint. If a new keeper only has one hive and loses it it is far worse emotionally than if one has 1 left. You can also rebuild the lost hive from hive 2 as long as it is strong enough. I also understand the cost factor very well. 1 other thing, use the same size boxes and frames for all, that way everything is interchangeable.