[cyberei]

I think this is could be a rather complicated feature. It's easy if your second phone is just a linked device like iPad or desktop client, but I imagine this might be very confusing for users. Now you have two phones with signal installed, but one has fewer features and if you lose the main device, you're screwed. This is unexpected for most users.

On the other hand, if the second phone should have the same capabilities as the first one, key management suddenly gets extremely complicated. For instance, each device has to be able to revoke others; what happens if the revoked device had granted access to three other phones, are they revoked as well? Can a device revoke it's "parent" device? And so on. I imagine they avoid this while they can.