Hi Jeff,
Why don't you try setting them up with Shared Lines and add an Intercom Feature so the Assistant can let the President know when there are calls/appointments for him;
For Assistants Phone - 7960
Line Button 1 - Ext 119
Line Button 2 - Ext 116 (Set to Ring and Flash)
Line Button 3 - Speeddial to Ext 123 (Label Text Intercom)
For Presidents Phone - 7960
Line Button 1 - Ext 116
Line Button 2 - Ext 119 (Set to Flash Only)
Line Button 3 - Ext 123 (Label Text Intercom)**Set this to Auto Answer on Speakerphone**
Hope this helps!
Rob