Unable to sync shared mailbox simultaneously from more than one iPhone devices using same user account.

1. Active Sync users are not able to sync shared mailbox simultaneously from more than 1 iphone device if they use common user account.
2. If they try to sync the same shared mailbox from one device at a time, it works fine on all the iphone devices. Only simultaneous sync requests are having issues.
3. Wants to now if this is a limitation of ActiveSync which is not allowing shared mailbox to sync from multiple iphone devices using same user account.

Exchange server used is Microsoft Exchange server 2010 sp3 Update Rollup 5.

Quick response will be appreciated!