For such bookings, the baggage allowance of the airline that operates the longest flight within either your outbound or inbound journey will apply. For example, a multi-destination journey from Abu Dhabi to Bali (with transit in Jakarta) and return from Jakarta to Muscat (with transit in Abu Dhabi), the following baggage allowance is applicable:
Outbound: Abu Dhabi to Jakarta (operated by Etihad Airways), Jakarta to Bali (operated by codeshare partner). The longest flight time is Abu Dhabi to Jakarta, so Etihad Airways’ baggage allowance is applicable.
Inbound: Jakarta to Abu Dhabi (operated by codeshare partner), Abu Dhabi to Muscat (operated by Etihad Airways). The longest flight time is Jakarta to Abu Dhabi, so the codeshare partner’s baggage allowance is applicable.