Two 75W heaters wouldn't necessarily be as good or better than one 150W heater. If they are actually both set at the exact same temperature, that's great -- especially if you don't have much water current, because it'll heat the tank more evenly. It's difficult to perfectly synchronize heaters, though.
Say you actually have one 75W heater set to 78F and one 75W heater set to 80F. Now winter rolls around and your house's ambient temp might be significantly lower. Once the tank gets up to 78F, the first heater shuts off, then the 2nd heater is running constantly by itself to try and get the rest of the way to 80F.
If they're both quality heaters, this might be irrelevant ... but still, if one heater is doing all the work and one is idle, this isn't really an optimal setup.
In the end, I'm sure dual heaters are perfectly fine and even preferable to some, but you should at least use heaters that have a light to indicate when they're heating. That way you'll know if one is doing most of the work and you can synchronize them. I don't think the Stealth heaters have a light anywhere.