Don, I had sort of the same reaction... huh??? If you this, you can't to that, unless...

But if we look at the process....(which I left out)

There are 2 different users.
One lands during a specific process on tab A
One lands on tab B.
Within that process A can not go to B and B not to A.

I have to look at it from the perspective/context A to B or B to A (not and).

It depends on how the (more than complex) flow goes.
The "last" user in a specific process flow will have the final 'choice'.
And it will be sometimes the user of tab A and sometimes the user of tab B.

Like I told John already in my previous post, I still have the feeling something will go wrong somewhere sometime.
The client seems to be confident....
He obviously knows something that I don't know...yet