In my moment of madness, I booked a return WT, upgraded one way to WTP during the booking process as I decided WT return was too horrible a thing to do to myself, especially in case there is a distinct absence of the exit row seat.
I looked in the MMB and the baggage allowance each way said it 2 bags.

The same thing with one way ET, one way CE booking - 2 bags for both direction.

So, as long as your flights are on a single ticket, it appears that the higher of the baggage allowance applies to all sectors.