There are a lot of beekeepers that have been doing this the old fashioned way for a long time and see their way as the only way. The only real difference between a traditional hive and the flow hive is the extraction process. The flow frames allow you to do it right there on the hive. No need to pull off the honey supers and then cut the wax off the frame then place them in an extractor to harvest the honey. The flow frames work out well for someone who only has a couple hives. Once you start getting to the point where you have dozens of frames to harvest the extractor makes that process much more efficient.
I think a good analogy would be with dairy cows. I am sure there were a lot of farmers that were reluctant to use the new milking device instead of doing it themselves by hand with a bucket.
To answer your question about starting with one flow and one traditional that is what I did. It worked out well. Ultimately I wholeheartedly agree with starting with two hives. It allows you to pick up on the differences between hives and if one is not doing well and the other is you can use resources from the one to help the other. I had one hive swarm on me and then the supercedure cells that were created did not return a mated queen to the hive. I ended up ordering a queen and using a frame of brood from the other hive to help it along until the new queen got producing again.
John