Warp Charges: A psyker model gets a number of Warp Charges equal to it's Mastery Level.
According to the GW official Eldar FAQ, Farseers are Mastery Level 1
Ergo, Farseers get one Warp Charge per player turn.
Spirit Stones states that a Farseer with them can use two psychic powers per turn.
No where in the FAQ does it state that Spirit Stones increases mastery level or grants more Warp Charges.
So, at current it seems like spirit stones basically serve no function (A common occurrence when an edition changes.) So, while it may give the Farseer the ability to cast more than one psychic power, there's only one Warp Charge in the proverbial tank to fuel them, so it's moot. (Unless there's some 0 Warp Charge psychic powers out there.)
So, are spirit stones currently non-functional on anyone but Eldrad (Who has the Mastery Level to pull it off)? The Eldar certainly wouldn't be the first codex to suffer from an edition change, but I am curious.