If you join an in progress instance in order to share the same lockout you need to kill a boss. So just being in that instance doesn't get you the same lockout
Nothing you have said so far counters what I am saying...
Ok, a more detailed explanation...
4 characters: Healer Nr 1, we'll call him A; Healer Nr 2, well call him B; dps, C and tank, D.
We'll use only EV in this example...
A goes into the raid and does everything up to Soa, then A leaves the raid.
A is therefore locked out of everything except Soa.
B then joins the raid and does Soa, therefore finishing the raid and getting the comms and whatnot.
A, who is locked out of everything but Soa then starts a new raid, which starts right before Soa.
A then leaves and C comes in to finish the raid.
A then starts yet another raid and promptly also leaves it to get replaced by D, D then proceeds to finish the raid.
A then starts a raid for the last time and finishes it himself.
Therefore A did everything up to Soa, B, C and D then do Soa, and then A finishes by doing Soa.
One time the lead up, 4 times the pay off...