Two undersized heaters are safer than 1 oversized heater. If a failure occurs on a heater that's oversized (whether it's stuck on, or off) the fish will suffer quickly (especially in a 29 g.) With 2 undersized heaters, one failing is not a disaster, just an inconvenience. And I am racking my brain, but I don't think there is any electrical cost difference of any significance. Watts are watts: it doesn't take more watts with 2 heaters than one heater, and the watts don't cost more if they come from different heaters, and the watts all work the same way in the tank.
So, within a range of temperatures, the heater will raise the water above the ambient temperature of the room (allowing for whatever extra heat is coming in the tank from a pump/filter and what is being lost.) Roughly speaking, if the heater can raise the tank 20 degrees, it can do so whether the room is at 62 or 78. If the heater dies, the tank either falls to 62, or if it gets stuck at on, it rises to 98. Having 2 heaters each which is only capable of raising the tank 10 degrees, allows for one failure and the tank will be at a low of 72 or high of 88.