how can I see shared contacts also in the drop-down list when I click on the "to" button

hey,

we have exchange 2010.

I have 2 users that share contacts...I gave permissions and all.

I acually can see the mailbox..contact...calendar...

but when I click on "To" I cant see the other user contact in the drop-list.

how can I add the contacts to the Drop-Down list.