This is bad advice. People grossly underestimate the effort to learn ONE language, never mind two related ones at the same time.
You will probably not succeed at learning a new language unless you have a pressing, extrinsic reason to. Self-study language courses have dropout rates north of 98%. Even in an educational setting with young people (better at learning languages than older ones), it's only about 20% who progress beyond the bare beginner stage.
Esperanto is MAYBE easy enough to learn from only curiosity and dedication... but I am aware that "eternaj komencantoj" is an expression in Esperanto-land.
If the goal is to be able to have a rough understanding and be able to make yourself understood, then I'm not so sure.
My French is passable, and it made picking up enough Italian, Spanish and Portuguese words that I can slowly work my way through many types of text in those languages a lot easier. Not least because my French teacher made a point of explaining how French vocabulary related to Spanish and English - just the habit of recognising which sounds are related in different languages helps tremendously in recognising words without having to explicitly learn them in each language. I don't use any of that much and certainly not enough to justify spending a lot of time learning to speak more languages flawlessly, but it's been helpful in many occasions.