The booking classes must match on the same PNR but can be mixed over legs. So:

PAX1 and PAX2 outbound in H and inbound in N would be OK (subject to any weird fare rules on combinations)
PAX1 outbound in H and inbound in N, and PAX2 outbound and inbound in N is not allowed on the same PNR

It's not an online/offline thing. It's the same reason why you can fall foul of availability problems as buckets run dry (e.g., if a flight is showing J9C9D9R1 and you try to book a CE flight for 2 people you'll both be put in D rather than 1 in D, 1 in R).