It may be different for ID tickets, but for normal tickets PNRs and ticketing can of course be totally separate. I have often had tickets with 17 segments on them (OneWorld RTW tickets) with all but the first one or two segments issued simply as open-dated. when I want to use the segment I make a reservation (which generates a PNR) for that segment. There may be 5 or more unlinked PNRs in existence at any one time for journeys for which I will use coupons from that ticket. Depending on the airline policy there may or may not ever be a ticket number associated with the PNR.
But this doesn't matter, because the baggage allowance is defined on the ticket coupon, and if it passes through North America then all coupons on the ticket have the piece-based baggage allowance.