Need help balancing a Homebrewed Item

[Hats]

Okay, so I've run into a problem with a NPC, namely she (Shabti Bard) is an Illusionist, but runs into the " can only concentrate on one spell at a time" rule. So... I thought about making a magic item to allow someone to concentrate on multiple spells. Obviously that would very dangerous, especially if the PC grab it somehow. In short, I would like some help with balancing this magical Item, which I've named Amulet of Many Minds.

Amulet of Many Minds
Aura Strong Illusion and Enchantment; CL 12
Slot Neck; Price 42,000 gp; Weight 1lb
Description
A dull metallic amulet with various jewels inlaid, each jewel different in size, color, and type than the other jewels. While wearing an Amulet of Many Minds, the wearer can activate it as a free action. Doing so causes the otherwise dull metal to appear polished, with each jewel shining and flickering as if reflecting the light from a fire. While the Amulet is activated the wearer can concentrate on multiple spells, equal to half her caster level (minimum of 1) plus one (maximum of five spells at any time). Also, while activated, the wearer take a penalty to will saves equal to twice the spells she is concentrating on. This penalty is alway equal to the maximum penalty that the wearer has incurred for the most recent period of time the Amulet has been activated. This effect can be used indefinitely, but for every hour the amulet is activated the wearer must pass a will save with a DC of 10 + Hours activated, or the Amulet automatically becomes inactive for 8 hours.
Construction Requirements
Craft Wondrous Item, Combat Casting, Uncanny Concentration, Creator must be able to cast 7th level spells; Cost 21,000 gp

Okay, so there it is. If you need more details, or have criticisms, please ask and share them.

[avr]

So what does it do exactly? The limiting factor of concentration is the standard action required which conflicts with both concentrating on another spell and with spending a standard (or greater) action to cast another spell. Does the NPC use quickened spells to get around the latter, or does it help with that too somehow?

[bookguy]

One thing that you could do to get past the standard action requirement is to allow the casting of another spell while concentrating (in lieu of spending a standard action to concentrate) but require a concentration check for each ongoing spell while casting the new one. The concentration check can be harder if the caster is concentrating on more spells simultaneously.

Also, I like the item, but if you are worried about shenanigans if the PC's get their hands on it, you could make a homebrew feat or alternate class feature (unavailable to the PC's or with very stringent prerequisites) that allows this multiple concentration effect.