Individual humans do make off much better when they get to select between products from competing companies rather than monopolies, benefitting from companies going out of their way to demonstrate when their products are verifiably better than rivals’. Humans get treated better by sociopathic powerful politicians and parties when those politicians face the threat of election rivals (e.g. no famines). Small states get treated better when multiple superpowers compete for their allegiance. Competitive science with occasional refutations of false claims produces much more truth for science consumers than intellectual monopolies. Multiple sources with secret information are more reliable than one.
It’s just routine for weaker less sophisticated parties to do better in both assessment of choices and realized outcomes when multiple better informed or powerful parties compete for their approval vs just one monopoly/cartel.
Also, a flaw in your analogy is that schemes that use AIs as checks and balances on each other don’t mean more AIs. The choice is not between monster A and monsters A plus B, but between two copies of monster A (or a double-size monster A), and a split of one A and one B, where we hold something of value that we can use to help throw the contest to either A or B (or successors further evolved to win such contests). In the latter case there’s no more total monster capacity, but there’s greater hope of our influence being worthwhile and selecting the more helpful winner (which we can iterate some number of times).
This happens during fine-tuning training already, selecting for weights that give the higher human-rated response of two (or more) options. It’s a starting point that can be lost later on, but we do have it now with respect to configurations of weights giving different observed behaviors.