i don't have a tape, so I can't check, but I don't think they actually show that the two groups meet.
so there is also the possibility (admittedly not very likely) that the two events (Ana shooting and Shannon getting wounded) are simultaneous but unrelated.
In support of this view, it seems that the tailers group should take more than one day to reach the swan station group or they would have likely met already.
Also, shannon and sahid where yelling, but the tailers didn't hear them (just the whispers) and i'd guess mike would be responsive to hearing shout "walt".
As far as sahid and walt, it was purposely ambiguous, so it could be either way