Relates to MC-184629 and MC-184619. Only one piglin is allowed to ride on a boat, it is not possible to have two piglins riding on one boat.
MC-172255 Piglins immediately dismount any entity that they are riding
MC-184619 Piglins riding on other entities float
MC-184629 Piglins riding on hoglins or other piglins automatically dismount them