It's been a while since I was through this so I am not sure if I have the details exactly - maybe this will help. If not, call the tech support at Wesbell (if that is where you purchased) they are great.
On the 6757, do you have Line 1 (in Administrator) defined as the persons name and Line 2 defined as Receptionist? If so, redo and reverse so that Line 1 is the Receptionist and Line 2 is persons name.
If you go into the Advanced Settings button web interface from the Administrator Phone settings and look at the Advanced Settings/Line x detail you will see that if you have 2 lines defined (like persons name & receptionist) that lines 3 through line 9 will repeat whichever identity you have specified for Line 1. Only that identity can handle more than one call on that phone at a time. So, if you want Receptionist to be able to handle more than one call, you need to make Receptionist be Line 1. The corollary to this is, if the Receptionist can handle more than one call on her phone, she can only handle 1 call to persons name at that call.
HTH
Regards,
Allen