Is "with" optional after "conferring"?
Possibly you are thinking of 'consult' or 'consult with'? Either of these is possible in the examples you gave. You can say that two people conferred, or that one person conferred with another.
Confer has two meanings:
Confer (with)
to exchange ideas on a particular subject, often in order to reach a decision on what action to take:I need some time to confer with my lawyer.
Confer (on or upon)
to give an official title, honour, or advantage to someone:An honorary doctorate was conferred on him by Columbia University.
Confer (Cambridge Dictionary)