It's generally a good bet to double up, so as to reduce the load on one heater and create a backup in case the heating element on one of them fails. In your case, however, if the max it will ever have to heat is 4 degrees or so, you'll probably be fine. If you wanted, you could put the Tetra heater in there and set it a few degrees lower, just in case. In theory, it should never come on unless - for some reason - the ambient temperature drops and the Jager can't keep up. But the Jager is excellent.