If you want to save money by not getting another bottle heater simply close the valve on the other bottle. when the first bottle gets low close it and open the other.
I think that would be the most cost effective way to run a two bottle setup. If you decide to go with the dual heaters check out ours, Its easily one of the best on the market in terms of getting the PSI up quickly and retaining the heat. http://www.harrisspeedworks.com/shop...cat=269&page=1