How much "more strain" on the heater would having another one do, in comparison to just having one to heat the whole tank? Surely having just one heater in a tank would be more strenuous for the heater?
Infact, I would probably set the the secondary heater at a slightly lower temperature than the primary so that you're not using twice as much electricity (in the case of a backup heater).
there is a good argument that, switching on an item, causes the most damage in, any, units life. so i think the "strain" argument has legs. though, to me, its an irrelevant point. i use two heaters to, potentially, save my fish, not my money!
i do, though, set my heaters as you suggest.
Yes but a single heater that is too small to heat the tank properly (by itself) would put quite a bit of strain on it wouldn't ya think? Thus leaving it more likely to burn out? Then you also have to worry that each part of the tank is a different temperature?
but, if you have two, the "second" heater would take up the slack. if you set as i do, both heaters would be on till the tank temp reached 1-3f, off of the temp i wish. only then would the "first" take any strain.
something else worth thinking about too. many 50ukg tanks could be heated, with 150w heaters. truth is, many of us could switch our heaters off. reducing, to a minimum, the wattage you have. can only help, both with cost and safety.