The best engineers Meta can afford worked on this revolutionary feature.

Probably, I mean the reason most people pop a 2nd SIM in their phone is for registering on websites where they’d rather not have their main number cross-referenced and datamined. This can be anything from Amazon to personal ads sites.

But the moment you give in to the temptation to register than 2nd number with Whatsapp Meta will be able to figure out from your contacts who you are and associate it with the main number.

The owner of what’s app is enough to keep me off it.

Not necessarily, they’re also allowing people with two single-sim phones to associate both numbers with the same WhatsApp account.

Which like the other comment said makes me suspect they’re really after collecting people’s 2nd numbers, in whatever shape or form (2nd phone, 2nd SIM, 2nd eSIM etc.)

work/private?

That’s called app cloning and it’s accomplished by the phone manufacturer keeping two copies of Whatsapp that each see a different phone number. Presumably you have a dual SIM phone.

and your phone only has one number… right?

Dual-SIM phones are a thing and have been for a LONG time. They’re not as common in the US (where I’m assuming you’re from given you aren’t familiar with dual-SIM phones) but even my S22 supports it, and I think even iPhones come with it stock now. A lot of people use two SIM cards for a multitude of reasons, and so many people have two numbers. Heck, some phones even support three SIM cards.

Dig they bring out a PC client yet btw?

It’s been out for a long time now. You have to set it up on your phone first but then you install the application on your computer, scan a QR code to link it, and at that point you can just disable notifications on your phone if you don’t want them.

Phones can have double sims, and WhatsApp web (been out for years by now) can be used on any decent Desktop Browser, it can also be installed as a PWA and I think there is a WhatsApp Desktop version as well.