Where does the law require both types? I'm not aware of any housing codes specifically requiring photoelectric types, and any house I've looked at, including mine, came with purely ionization types. Though it's been a few years, and it may have changed recently - this is less of a niche concern lately.

As for dual sensors and gating... do you actually trust your life to "nobody will admit what algorithm they use"?

My house has all the smoke detectors wired together (they're on an AC circuit, with battery backup, with a signal line running between them all), so I have some photoelectric and some ionization, depending on where in the house they are.