The logic for 2 smaller heaters is that when one sticks ON, it won't cook your tank. Use the manufacturers reccomendations for your tank size, divide by two and get 2 of that size heater.
If you are using a controller (AC, RK, etc) to run your heaters there isn't a need to run 2 heaters since if the controller goes haywire, both heaters will be on anyway, unless, of course, a single one is too small for your system. To achieve an extra level of safety if you are using a controller, set the built-in thermostat on your heater(s) to a degree or two above your controller set point so that if your controller sticks on, the thermostat will turn off the heater.
I use 4 Visi-Therm Stealth Heaters in various parts of my system (overflow, sump, refugium) all controlled by my ACII.