The two iPhones need to be using different Apple IDs, configured as the devices to determine the location for those Apple IDs, and both Apple IDs need to be part of the same family under Family Sharing. Then you should be able to create this Home Automation:
they've got all that, but you've missed a major point that they've mentioned twice.
There's a third device, the iPad used as a home hub, on it's own account.
That iPad is "always home". So the "when last person leaves" or "when no-one is home" automations never trigger, because even if "person 1 with iPhone" and "person 2 with iPhone" both leave the house, "Person 3 with iPad" is still at home.
they don't have to be part of the same family, you just have to share the home with them, and enable access when away from the house, so automations can run when they leave
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Have you tried turning off location services on the iPad? (main settings app -> privacy)
not sure if that will help
I believe for homekit purposes apple intends for the iPad and your iPhone to be on the same account, so if turning off location services doesn't help, you've got 2 options
1 - sign the iPad into one of your accounts, but you can restrict other apps and messages if you want to for privacy, it might be fairly easy to re-enable, do depending on how well you trust the other people in your house, that might not be a good solution. You could also find some way to physically lock the iPad in a drawer or box, while still being plugged in.
2 - get another home hub, either a HP mini, or appleTV.
the mini at $99 is your cheapest option, and with homekit, having a device where you can just yell at siri instead of picking up one of your other devices comes in handy. they occasionally go on sale for a bit less, or you can also find refurb units.
with the appleTV, you do need a TV to setup, there are refurb units for a bit less if money is a problem