The thing is that, as you noted yourself, the analogy doesn't hold, for people are not AWS servers waiting for charge or failovers. If they chose to work 20h/wk, it's presumably to spend their time for other pursuits, eg. family, and they often won't be able to or won't agree to work more when their employer need it, perhaps unless it is planned well in advance (eg. Black Friday period).
> Basically, the company is renting two brains for the price of one.
Or, as I questioned, you might got two partly focused and perhaps less skilled people thinking about your problem, instead of someone more skilled totally focusing on it. As was brought in another comment, if you got two freelancers, then yeah, you could pick two brains, but if you have two family guys, not so much.