That's still one device. If you turn the primary phone off, the secondary device stops working. WhatsApp just proxies everything through the primary device, it's like WhatsApp Web.
They used to, but that hasn't been true for a few years now.
Now it uses the Signal protocol's native multi-device capabilities, specifically in the "key per device" variant (unlike signal itself, which uses "key per account" if I'm not mistaken).
> You can now use the same WhatsApp account on multiple devices at the same time, using your primary phone to link up to four devices. You’ll need to log in to WhatsApp on your primary phone every 14 days to keep linked devices connected to your WhatsApp account.
It's not true for Signal either. Why don't you try it for yourself instead of spreading outdated (at best) information? Signal supports native multi-device capabilities without relaying everything through the "primary" device.
Messenger seems to be properly multi-device, but you pay for this by some PIN code bullshit (maybe they removed that, I haven't seen a popup about this for over a year now?) and having to sync chat history in the background, through a process that is, of course, broken and unreliable.
I'm actually still jaded about this. Messenger worked fine before they broke it by introducing E2EE; it took years for them to fix the problems this caused (at least the ones that were immediately user-perceptible).
yeah messenger still has the pin code thingy, i'm curious why they do it at all that way, can't you just have your keys on fb servers encrypted with another set of keys derived from your password, which is much stronger than a 4-6 digit key?
It's still broken if you're like me and you clear cookies
"Let's take people's years-long history between each other and just utterly break it. Why? 'privacy'" but they've never cared about it, they're opportunistic fucks. It's Zuckerberg's company to do with it "as he wishes" https://news.ycombinator.com/item?id=16770818