I never like to repeat what others say, but it is very true - having two hives allow you to gauge one against the other, which will quicly point out early mistakes that you or the hives are making - and make it much easier to correct quickly. A big issue is queen health or prolificity. You'll be amazed at the fast drawing out of comb in both hives if the queen is healthy, pheromonally controlant and prolific. A single hive gives you very little feedback, and operservation and knowledge is everything in the beeyard.
Two healthy queens, and equal number of workers with the same food sources and sugar-water supply SHOULD equally grow in bee count, brood count, foundation development, honey and nectar/pollen stores, etc..
If you notice any major differences in either hive, you can better correct a problem before it becomes a major one.
I've had a dozen hives and I've had only one - I'll gladly stick with 2 if given the chance to have hives for observational purposes. With just one hive (especially if new to the hobby) is not a good way to start, you just won't see any problems coming as easily as with two.