I think that restriction is only in a few countries, and only for commercial operations.
It's certainly not true in Britain.
But EASA standardisation is seldom all that standardised!
I'm sure you're right that there are some obscure conditions somewhere where it's possible to have 2 people logging in an SEP (other than the obvious of instruction). But it's rare, and certainly doesn't apply to the vast majority of pilots, the vast majority of the time.
G