why does it matter if your AF is due 8 days later than someone else.
It doesn't, the embryo will still be releasing HCG into your blood...

Why can't they both test on the same day? CD34? If they both had a baby implant at 7DPO? Why does the woman with the longer LP have to wait until CD40 to test?
They can... but the woman who's af is late has some evidence to test, so more reason to believe she is pg. The woman who is still waiting to see if AF is going to arrive has no evidence to suggest she is pg yet. That is not to say she can't test... but it may be a waste of a test because she has no evidence that she may be pg.

So if the baby implants at so many DPO, what on earth is it doing for the other number of days for the woman with the extra long LP? Once it is implanted the HCG is going into the blood stream.
The longer your lp the better chance you have for a sticky baby...

Does that help?