Can you tell me your use case?
If your use case is just simply to know whether a lead is interested in either demo or info, you can create a smart list to find them without creating another field.
If you do want another field, you need to figure out what you want to do in each scenario
Demo - True, Info True => Contact Me = True
Demo - False, Info True => Contact Me = True?
Demo - True, Info False => Contact Me = True?
Demo - False, Info False => Contact Me = False
Next create a triggered campaign to update the contact me field when lead sets new choices ( based on logic above)
Smart List - Data Value Changes for either the Demo Field or Info Field
Flow -
(1) If Demo is true => Contact Me = true
(2) If Info is true => Contact Me = true
(3) Default => Do Nothing.
(One time fixup) To set the Contact Me value based on your existing data value from info and demo field, create a batch campaign to do a one time job.