one condensing heater versus two cheaper power vented heaters
I own a duplex (live upstairs and rent out downstairs) and currently have two power-vented water heaters. One needs replacing now, and the other won't last much longer. The building no longer has a usable chimney, so atmospherically vented heaters are out. My question is, should I spend $750-800 each for two non-condensing, relatively inefficient power-vented water heaters, or should I buy one condensing heater, such as a Vertex GDHE-50 from A.O. Smith, or a Polaris from American Water Heaters?
I'm tempted to go with one condensing heater. From what I've read, it seems as though one of the condensing heaters would supply enough hot water to meet both units' needs. It would save space, and my understanding is that it would be cheaper to run than two lower-end heaters. On the other hand, with one unit all my eggs would be in one basket.
Am I overlooking important considerations? Thanks for everyone's opinions.