Yes, if you want to use 2-for-1.
Or you can buy 'dummy' train tickets from eg Queenstown Road (QRB) to Vauxhall (VXH) inbound dated for the first day you need 2-4-1, and an outbound single VXH-QRB dated for the last day of your visit. You can get these from any station but you will need to ask the booking office clerk not get them from a machine.
Note that 2-4-1 will be valid with these 'dummy' tickets at all attractions for which a train journey into central London is appropriate. They won't be valid at attractions "behind" Queenstown Road, where you would need to travel in the opposite direction (e.g. Hampton Court, All England Tennis Club at Wimbledon and Kew Gardens). Another decoy rail ticket (or actual ticket for out of zone travel) would be needed for each of those.
Thank you.