I'm a little lost...... if you have a single PNR, your entire ticket would either be 014 or 618....not a combo 014/618..... either way, under IROPS gate agents can help you book other flights..... Whether they need approval from the other airline is an internal issue between AC/SQ themselves (i.e. AC may need to call SQ for approval), and some airlines would prefer to book their own carrier first (i.e. UA)

I can tell you from experiences:

YYZ-ORD (AC) / ORD-NRT (NH) / NRT-TPE (NH)..... AC went MX and went book us AC1 direct to NRT
SYD-YVR-YYZ (AC)..... Due to WX issues out of YVR, went and book my friend on UA
NRT-EWR (UA) / EWR-YYZ (AC)..... Due to hurricane sandy, went and book me NRT-SFO (UA) / SFO-ORD (UA) / ORD-YYZ (AC)