I’ve done that to a few engines. That is a Lionchief engine so you would need to solder chokes on each side of the motor and then I recomend wiring the motor in series. Make sure to check the direction of motor rotation when you do. You have to wire the new truck opposite the polarity of the existing truck.
I’ve always loved the Lionel RS-3’s, great simple engines (and affordable, lol). Did a TMCC conversion to a single-motor Southern Pacific RS-3 about 10 years ago. Installed Railsounds and speaker into a 2nd one, then slaved it to the first unit with the TMCC board. They run pretty smooth together, but never thought to add weight as others here pointed out. Most I’ve tried was a 6-car consist, worked well. I have the new John Deere RS-3 as well, and agree it’s a limited puller. But my 4x8 layout is limited to 3-4 car trains anyway, so not that big a deal for me.
Going to do a TMCC conversion on a dual-motor UP RS-3 #8805, will let you all know how that goes.