FYI when you order a WR51x10031, it automatically subs over to a WR51x10101, the 10031 they no longer make.

I beleive that the "same part number" you are referring to is the number found on the molded plastic end caps. This part number is for the end caps and not for the heater elements. The dual element setup is not simply two of the original elements put together. It is two lower power elements in series. I wouldn't reccomend the use of a resistor in place of one of the heating elements in the dual heater. It may work (poorly) with the proper resistor and/or inductor value, but you would only have one half-power heater working for you. This dual arrangement was likely an improvement to the single because the single pulled too much power and burned out too soon. By replacing this setup with two lower power heaters, the assembly provides about the same amount of heat and uses about the same amount of power overall but each individual element uses less power and therefore will last longer.