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.
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.