I always like to use 2 undersized heaters on tanks.
The most common malfunction (other than simply stopping) is to get stuck in the on position, and cook the tank.
With 2 heaters, if undersized, hopefully the other functional one will shut down, and even constantly on, the bad heater can't overheat the tank before its malfunction is noticed.
I had a Hydor 300 get stuck and contort a PVC pipe into a bend, the temp in a 300 gal system reached into the 90s when I noticed.
Not to single out Hydor, I've had many other brands of heaters get stuck over the years.
Unless you use a heater controller, 2 undersize ones provides some insurance.