In case anyone is interested, the phone plus web combo was apparently the final solution to the thorny problem of establishing identity with the web client without requiring any more info on the user beyond the phone number.

All bets are off, now that it's FB's beast, but the two device solution was intended as a privacy feature, afaik.

I sat next to Pasha Sadri, the web client lead, for 4 months before the acquisition. He mentiond the problem a few times.

The fact it took another 2 years to get the client out seems to indicate that multiple problems were solved in other ways.

It doesn't have to be on the same network, only on the internet and you need to scan the code (see also my response above)

Today I learned - thanks!