There was no forkroot in those cases. Egwene says she has no hope of breaking Silviana's shield, and Katerine wants to have Barasine alone hold Egwene's shield during the night, when there would be no forkroot. Silviana makes it clear that the only reason two women are holding Egwene's shield is because she thinks Egwene is "tricksome". No one thinks she's too strong for one Aes Sedai to hold.