I'm guessing they are on the same circuit. If they are both on the same 15 amp breaker that might be a bit too much. Also, you should check what else is on the circuit. If for example, if the electric element for the water heater is on the same circuit too, that could easily put you over.
If none of that pan's out, I would check the connections for each fireplace to make sure they are tight and good.
Finally, if you are comfortable working with electrical systems, you could investigate if it is possible to move one of the fireplaces to the other leg of the 50 amp service.