Motorcyclist Stephen Probert died at the scene following the collision with a BMW on the A470 Betws-y-Coed-Llanrwst road at 13:30 BST.
His partner Joanne Winder, who was riding pillion, died on Tuesday at Ysbyty Gwynedd in Bangor. Both were from the Colwyn Bay area.
Two other casualties were taken to hospital following the collision.
North Wales Police have appealed for witnesses and are investigating.
A second person has died after a crash in Conwy county on Easter Sunday.