To allow your visitors to book a return journey, you have to activate this in the order settings.
This setting can be adjusted via the "My Products" / "Product Settings" / "Order Forms"
You can adjust allow return bookings under paragraph booking options.
As an additional option you can add discounts return bookings You can create a discount through the menu, product settings / Miscellaneous / discounts. You can specify in the discount settings that these are only used for return bookings.