Controlling the power through a heating element?
You can't get the power down to 200 W with resistors unless those resistors can handle their share of the power. In other words, the resistors would need to be more heaters. If your heaters are 300 W, putting two in series would double the resistance and halve the power.

Or you can find another resistor (a power resistor or heater) which will bring you to exactly the power you need: at 120 V, your target current will be 1.66 A. The resistance that will give you this is 72 Ω. Your current heater is 48 Ω, if the given wattage is accurate. So you need to add an extra 24 Ω of resistors/heaters that can handle 1.66 A. The power will be I²*R, and if you split it over two resistors, metal power resistors should be able to handle that.

Another way to arrive at the exact power output you need is to cut out part of the waveform with a solid state AC dimmer (I think something based on a triac). The peak current output will still correspond to a higher power than your power source is rated for, but the average power can be tuned to be the power you want.

I'm not sure whether AC can be dimmed with PWM, but if so, that's also an option analogous to using the triac.