According to a report on the Kent Live news site at http://www.kentlive.news/news/kent-news/reporter-tried-buy-season-ticket-1013222, and on the Sevenoaks Rail Travellers' Association site at https://srta.org.uk/wp/posts/2843, Southeastern commuters from outside London can legally make savings by buying split season tickets from their local station to New Cross and New Cross to London Terminals (or a Travelcard starting at New Cross).
One of the comments posted on the SRTA site suggests that you may be able to load both tickets onto The Key smartcard, which can hold up to five tickets at a time. But would this cause problems if you did not alight, exit and re-enter the station at New Cross? If, for example, you went straight from Sevenoaks to Charing Cross with such a combination of tickets loaded onto The Key, you would sign in at the ticket gate at Sevenoaks with your Sevenoaks-New Cross ticket, but not sign out with that one, and on arrival at Charing Cross you would sign out with your New Cross-London ticket without having signed in with that one.
On the other hand, ticket office staff are sometimes reluctant to issue such a combination of tickets, although it sounds as if the word is getting around. Because of this, a better bet might be to order both tickets online via the Southeastern website and opt for paper tickets (or use an Oyster card for the Travelcard leg of the journey, although I'm not sure what happens if you have a Travelcard loaded onto an Oyster card, and sign in at the start but don't sign out at the end of the journey, or vice versa).
One of the comments posted on the SRTA site suggests that you may be able to load both tickets onto The Key smartcard, which can hold up to five tickets at a time. But would this cause problems if you did not alight, exit and re-enter the station at New Cross? If, for example, you went straight from Sevenoaks to Charing Cross with such a combination of tickets loaded onto The Key, you would sign in at the ticket gate at Sevenoaks with your Sevenoaks-New Cross ticket, but not sign out with that one, and on arrival at Charing Cross you would sign out with your New Cross-London ticket without having signed in with that one.
On the other hand, ticket office staff are sometimes reluctant to issue such a combination of tickets, although it sounds as if the word is getting around. Because of this, a better bet might be to order both tickets online via the Southeastern website and opt for paper tickets (or use an Oyster card for the Travelcard leg of the journey, although I'm not sure what happens if you have a Travelcard loaded onto an Oyster card, and sign in at the start but don't sign out at the end of the journey, or vice versa).