Normally ACE will only let you transfer one call at a time to an extension, i.e. the next call will be queued. If you know that a receiving party can handle multiple calls simultaneously you just add the telephone number to a sort of white list, in the Function numbers window, found in ACE Admin, under menu choice Media | Telephony.
To administrate function numbers you need the access function Own function numbers in your access role.
In the Function numbers window you add telephone numbers leading to functions outside ACE, such as ACD queues, hunt group or IVRs, together with a comment about each destination. Save with OK.