I think I may have stumbled on a way to farm these without harvesting Darkwraiths all day. It's similar to farming vertebra shackles with a friend, but with a twist.
The person who wants Pale Tongues needs to equip the Way of Blue covenant.
That's the major difference. From there everything's the same: once WoB is equipped, drop your red sign for a friend to summon you as a dark spirit, kill your friend. A Pale Tongue will drop.
I haven't tested with the allied covenants (Blue Sentinels and Blade of the Darkmoon)--WoB has worked every time.