I've heard of using two if you have two canister filters on a large tank. Using two in the same line means the first will be doing all the work, the second will sense the warmer water, and hardly run at all. I guess you could set the first one a little cooler, sort of a pre-heater for the second.
The idea behind using two conventional heaters in a tank is to prevent cool & hot spots, though this isn't a problem with adequate circulation. It's also a safety measure, using two heaters that add up to the proper wattage for the tank. If one goes down & doesn't heat, the other can keep the tank at a reasonable temperature. If one sticks on, the other will cut out. With the one undersized heater constantly on you have a lesser chance of "boiled fish"