From experience, I can say that if your cat is really determined to slip out, he absolutely will (cats are liquid, after all), so it’s more about risk management: ensuring that the harness fits well, that he’s used to it and feels comfortable in it, that the harness has positive associations for him (“harness = I get to go fun places! no harness = I’m stuck inside”), and that he’s properly supervised while wearing it. The last one is especially important, because it includes doing your best so that your cat won’t have reason to want to slip out. Once used to the harness, the main reason a cat will try to slip out is sudden fear that prompts their flight impulse, so you have to be aware of your surroundings and proactive at either removing him from fear-inducing situations before they arise (this means monitoring both your cat’s vibes and the environment around you), or being ready to pick him up and put him somewhere secure or hold him tightly in the case of something sudden and unexpected (like a loud truck passing by). Not all cats have the temperament for going much farther than the backyard or around the block, and so knowing your cat’s limits is also important.

As far as double-harnessing, I’d be worried that it would make the experience uncomfortable for him (and thus make him more prone to wanting to slip out). Generally if you can pick your cat up by the back of his harness and he doesn’t fall out, it’s probably as secure as you’re going to get. I haven’t used your brand of harness before, but it looks more secure than the strappy kind.

tl;dr: to prevent break-outs, as long as the harness is properly-fitted, the most important thing is to ensure your cat is comfortable and very used to the harness.