greg765,
Selecting CONT or FLT on the start system switches CONTinuously fires BOTH ignitors L and R irrespective of what ignition system selected for as long as you have CONT or FLT selected when engines are running.
Selecting BOTH on the ignition system switch enables BOTH ignitors (L and R) for the engine start sequence only. NB...EEC will operate BOTH ignitors (L and R) inflight automatically if it sees a possible flameout condition irrespective of which ignition system is selected L,R or BOTH. Hope this helps.
Rgds McHale.