Is this what kalleboo is referring to https://kyivstar.ua/en/mm/services/voice/sim_pair ?

> Incoming calls, messages etc. can be taken only by one of the mobile terminals, in which SIM-card is in so-called “active” state. Accordingly, SIM-card in another mobile terminal is passive and any outgoing operations can be made from this device.

> To transfer basic SIM-card into active state it is necessary to dial 1071#. combination on mobile device keyboard. In this case extra SIM-card automatically transfers into passive mode. To transfer extra SIM-card into active state it is necessary to dial 1072#. combination on mobile device keyboard. In this case basic SIM-card automatically transfers into passive mode.

Yeah, I can understand why Apple didn't go down this path.