Technically, I believe it does.
Assume a unit (farseer) that is within 12" of two flyrants.
The rule states:
Enemy Psykers within 12" of this model have a -3 to leadership
Lets start at the first one.
Is the farseer within 12" of Flyrant1? Yes. -3 Leadership.
Is the farseer within 12" of Flyrant2? Yes. -3 Leadership.
There is nothing that states that this effect is something that can't stack. You need to check two conditions. Is it within 12" of a model with that rule. AND. Is it a psyker.
That's obviously not RAI as three warriors can give -9, but that is what the rules say.
actually re-reading this
Shadow in the Warp: Enemy Psykers within 12" of this model have -3 Leadership.
what that really says is your psyker is now -3LD, but since you cant go below 0LD its just 0 and they can only pass on double 1s.