I think I’d check the anode rod in each one and see which one was most consumed. If they are about the same, it’s a wash, but if one is less consumed than the other, that would be the heater I’d keep using. Presume you’re going to have to replace both anodes, as Bradford’s have such a short nipple that it’s impossible to remove without wrecking the threads.
But it wouldn’t surprise me to find that one tank is better than the other because it’s easy to get a parallel setup out of balance, and then one tank does all the work.
Then it becomes a question of what to do with the other heater. I’d be tempted to try to sell it on eBay or Craiglist or something like that. If you change the anode once in awhile, there’s really no reason either tank should ever wear out, but you’ll have to do that with both of them or one of them will rust out eventually. If you keep the spare, keep it full of water, but with the gas off. If you ever need it, you can treat the water with peroxide before use.