Ah, internal heater 
Thought for some reason it was an internal filter you were speaking of.
In that case, no, do not run the internal heater for now until you know what your new external heater does temperature control wise.
Some people do have two heaters, in usually large tanks, both will work at same time to maintain temperatures and have peace of mind that if one heater breaks down then the other will keep temperature levels good until get a replacement.
But the purpose of an external is to have no unsightly heaters inside the tank
Thought for some reason it was an internal filter you were speaking of.
In that case, no, do not run the internal heater for now until you know what your new external heater does temperature control wise.
Some people do have two heaters, in usually large tanks, both will work at same time to maintain temperatures and have peace of mind that if one heater breaks down then the other will keep temperature levels good until get a replacement.
But the purpose of an external is to have no unsightly heaters inside the tank
