Shared Hold

So I noticed that PJSIP allows multiple registrations for the same extension.

Does this allow this scenario?
Phone A Extension 101
Phone B Extension 101
Phone A puts call on hold.
Phone B can pick up call.

No it does not allow that

In these cases you likely want to park the call

I know that parked is better, but I have a client that is stuck on the thinking of his old nortel system, where you can put the link on hold and pick it up anwhere.

That’s typically called SLA or shared line appearance. Something that asterisk does not support.